Engine Compression Advice needed

[Only Buicks]

I am just getting ready to drop the engine into my car. I want to check the compression before I install it to see if all is well (somewhat). What should I get for a reading on a cold engine? Are there any other checks I should make prior to installing it into the car? I beleive this engine was done some time ago and then just never installed in the car. I removed the head and the cylinders look good, no ridge. The valves look like they where probably re-ground. As the previuos owner passed away a few years ago I can't call him up and ask him how far he got and if there was anything else he needed to do to it. I have never rebuild an engine so I am not really sure what I should be watching out for.

Any help or suggestions would be appreciated.

[NTX5467]

In any compression test, cylinder-to-cylinder variation is probably more important than specific numbers. As with any other compression test, squirting some light oil into each cylinder before the test can bring the numbers up a little (if they don't come up that much, then that would usually mean less wear . . . or just the opposite, which we can hope is NOT the case).

As to the "freshness" of the short block, did you see any hone marks on the cylinder walls (the "cross hatch" pattern) or were things shiney? That could be an indicator of what might have been done. Also, sometimes oversize replacement pistons will have the oversize stamped into the piston's crown (top side), but not always. You can also roll it over on the engine stand and remove one rod cap and one main cap, checking for new bearings (also, you can PlastiGauge them for clearance at the same time) or wear patterns on the (possibly) old ones. Then you can lube, reinstall the main/rod caps, and retorque to specs.

Hopefully, everything will be as you suspect it should be.

Enjoy!

NTX5467

[Only Buicks]

NTX5467,

Thanks for your reply. The cylinder walls where clean and as I mentioned they had no ridge, but the cross hatch marks where not there. There where circular finish marks that would indicate that maybe they where honed by hand???? I measured the cylinder bores and they where 3.100 which from what I have read makes them 0.010 over. I did not drop the pan as I do not have a stand, the engine is just sitting on a dolly. I did take a compression test and got 60 lbs. on each cylinder all within 2- 3 points, no notable variation just very low as the shop manual calls for 115 lbs. At this point I am not sure if I should press forward and install it or get a stand, drop the pan and pull a piston. If I do pull a piston, then what am I looking for? To pull a piston is it as easy as removing one rod cap and then pushing the piston out the top and is the reverse how it goes back in or am I creating a bigger problem by doing this? Will I need any special tools to reassemble?

Lots of questions, your help will be appreciated.

Thanks,

Glenn

[Guest brh]

Just an idea since the engine is out how about doing a cyl leak down test. This is done by putting the cyl you are testing at TDC on compression stroke. Instead of a spark plug a fitting with a gage is installed. Take the compressor pump air into the cyl. If it looses air through the intake, or exhaust valve you have a valve problem, if it looses air into the pan (you should be able to hear it hiss). You have a ring problem. and last but not least if ya hear a hiss out of the water jackets ya got a bad head gasket. If the cyl holds air pressure its O.K. Hope this helps.

[JohnD1956]

Bruce, since the engine can't be run, won't there be some air escaping from the rings unless they can be expanded like when the engine would be warmed up?

JD

[NTX5467]

That would be a good idea, Bruce. I do believe, though, that it takes a "shop air supply" that will not only produce pressure, but also a certain volume of air at a particular pressure. There is a "tool" to do this test with. From memory, I think that if leakdown is less than (possibly) 10%, then things are fine. Still, this and the compression test will only test the integrity and seal of the compression ring and will not be a possible determiner of if the oil control rings are in good condition.

Other than the piston ring/cylinder wall interface, a side issue can be the "ring gap" of the piston rings. It has a particular spec (usually a set amount per inch of cylinder bore?) as with other parts of the rotating assembly.

Many engines would have ridges at the top of the cylinder bores (from piston ring wear on the bore itself, with time and miles), but some have less ridge than others. Also, there is a particular machine tool to remove said ridge to get the pistons out (other than the piston itself catching on it, the piston rings would).

If you measured the cylinder bore at .010" oversize, that would still be in the range of a "standard" or "standard, high limit" piston. Yet if the piston rings were fresh or still in good shape, that additional size should not really be a problem for normal street driving (with lower stress levels). It's also possible that there could be some piston "knock" when it's cold.

What could have happened is that the oversize piston rings were still available, but pistons were not (or were priced higher than the prior owner wanted to spend). Therefore, the rings could have been put on the original pistons and the piston skirts were possibly "knurled" to help the remain quieter. Then, rather than a traditional hone to rough-up the cylinder bore surface, it could have been done by hand with either some red Scotchbrite or similar--key thing is to put a pattern on the cylinder wall that can help retain an oil film until things get "worn-in" a little (in this case, the fresh rings). This might sound like a highly substandard "rebuild", but in prior times when money was tight, things of this nature were pretty common--and it worked well enough for what was needed.

If the compression values concern you, then you probably ought to either take the engine to a competent machine shop or buy a few extra tools and an engine stand. From this point on, things can get a little messy and involved to see what's really going on or you find a competent "somebody" to do the work for you. There will be "a cost" either way, just depends upon which way you desire to do things.

As for piston removal, you rotate the crankshaft to a particular point (usually, I believe, when the piston is at TDC), remove the rod cap, put some rubber hose over the exposed rod bolts, and then gently push the piston/rod assembly toward the top of the engine. Not to hard, per se . . . but there's more.

With the piston out of the engine, you can look for the knurling marks and also inspect that total piston/rod assembly itself. You can also do a more accurate cylinder bore diameter measurement and correlate that to the measured size of the piston. To put it back "in the hole", you'll need a piston ring compressor, which usually has a flexible band that you will use to compress the rings and then tap the piston/rod back into the block with the end of a hammer handle, or similar. There's a particular method to get everything in the same place and right time and hold your mouth right to getting the piston back into the cylinder bore. But it's not out of the realm of things if you have enough of the correct tools to make it happen. Also be sure to retorque the rod cap to the appropriate torque value and put some type of "lube" on the bearing surfaces.

In some cases, the cranking rpm of the starter can influence the values you get with a compression test. Not sure of that might be a side issue in this deal, though.

Just some thoughts,

NTX5467

[Guest brh]

Good question and I am going by memory many years ago. I don't believe warming the engine is necessary, but I could be wrong.

[Only Buicks]

Bruce,

Thanks for this response, it is very helpful and I appreciate your time. Funny that you mention the speed of the starter. The starter I used during the compression test is an old one that I purchased recently to have rebuilt. It was a little slow turning over coupled with the fact I was not using a fresh battery. I think the first thing I will do is pull the starter and battery off the engine that is currently in the car and bolt them on this engine and re-test the compression again. I'll let you know how everything goes.

[NTX5467]

Regarding the rings needing to be "hot" to seal better . . . the rings are designed to be "under tension" at all times they are installed on the piston and that assembly is in the cylinder bore. When the rings are on the piston, out of the cylinder, they are bigger than the piston's diameter and would be loose in their respective grooves in the piston. In order to install that assembly in the cylinder bore, a "piston ring compressor" would be used.

One type of piston ring compressor is like a wide band that encircles the piston and pushes the rings further into their grooves so that the piston/rod assembly can be tapped into the cylinder bore from above. The other variation is a ring compressor that has an internal taper. As the piston/ring/rod assembly is pushed further into the compressor, the taper forces the rings into their grooves (as the band-type compressor does) so that the whole thing will slide into the cylinder bore somewhat easier. This type of compressor is specific to a certain cylinder bore (basic) size, whereas the band compressor is universal (within a certain diameter range).

End resut is that, hot or cold, the rings have tension against the cylinder bore they are sliding against.

Having the engine at operating temperature would ensure that everything was closer to what the engine would see in an actual operational mode. Plus everything would be well-lubed with warm oil, for example, making things easier to turn over. But, using an "ambient temperature" rather than operating temperature would simulate another actual operating condition when the engine starts from "cold".

As the engine is out of the car, if you desire to get the engine asssembly heated before the compression test, you could run hot water through the water pump and block and such via your residence's hot water heater. It might take some rigging and plumbing, but most of what you might need should be at one of the home improvement stores. Just depends upon how far you might desire in this venture (adventure).

A "shadetree" gauge of compression would be to spin it over with the starter motor (with the spark plugs out of the engine) and then see how the speed of the starter would slow down with all of the spark plugs installed. Having a fully charged battery would be necessary, too.

Enjoy!

NTX5467

[Aaron65]

I think we're missing the fact that all his cylinders were at 60 psi. That's pretty low. I think something else is going on here. When I bought my '53, number 8 was at 0 psi. I hooked compressed air to it and listened to the carb, breather, and exhaust. It was hissing out the exhaust, and I had a bad valve (very bad, in the shape of an oval, and cracked!). The other cylinders weren't much better. I replaced all the valves, and now I'm at 105 psi cold, 110-120 hot across the board. Not perfect, but OK considering I didn't touch the lower end. You should hook some compressed air to a cylinder at top dead center and listen to see where it's coming from, because at 60psi, that motor probably isn't going to run too well. Maybe the starter isn't spinning fast enough, but make sure to pull all your plugs and charge that battery. When I leave all the plugs in and check compression, my values drop to under 100 psi, so cranking speed is an issue. Good luck!!!

[Only Buicks]

NTX5467,

Today I took the head back off, flipped the engine on it's side and removed the oil pan to get a look at the bottom end of the engine. It also was very clean and showed evidence of being worked on. Fresh looking oil pump, new lock nuts on the piston caps, no sludge, very clean. Looks like maybe new bearing inserts have been installed as I can see a shiny ring on both sides of all 5 bearings caps and fresh brass shims under each cap. I removed one of the pistons but I'm not really sure what a good or bad set of rings look like. All is very clean. The one thing that keeps haunting me is why there are no cross hatch marks on the cylinder walls? How long has this type of honing process been in use? In the old days was this process used? I guess I am at the point where I need to make a decision to either install the engine or send it out to a shop and have it gone through. Problem is I have this belief that the vast majority of engine shops might be very proficient rebuilding a 350 V8, but don't completely understand a 70 year old straight 8. I would certainly like to hear some opinions, thoughts or other peoples experiences on this.

Thanks,

<img src="http://forums.aaca.org/images/graemlins/frown.gif" alt="" />

[old-tank]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> measured the cylinder bores and they where 3.100 which from what I have read makes them 0.010 over. </div></div>

0.010 over size should measure 3.010...if this was a typo disregard, otherwise maybe this is the reason for the low compression

Willie

[NTX5467]

Glenn, as far as I know, the crosshatch hone pattern has been in use "for ages", including back when engines with 8 spark plugs were "straight" and not "bent". That particular pattern is for oil retention and oil control and other design issues. If it's not there, it's been worn away. Similarly, the cutter marks on the piston skirts and sides of the piston (horizontal in nature) are probably worn smooth with some vertical wear marks replacing them. If you might find some deeper, shorter "marks" on the piston skirts, that's where they were "knurled" to make them better fit the size of the worn cylinder bores (an earlier "fix" for pistons that we'd replace in more affluent and modern times), similar to knurling valve guides, except it's an external surface being knurled.

With everything being as clean as you describe, it's possible that the engine was "gone through" (and vatted in the process), but some particular wear parts might not have been replaced due to cost/availability issues for the shop doing the work or of the prior owner.

With the piston out of the motor, ease the rings off and then squarely place them in the cylinder bore about 1" below the deck surface. They should fit reasonably firmly in the bore, when compressed to be in the bore. What you'll be looking for is the size of the "end gap" of the ring, plus that it has enough tension on it to stay put when it's squared-up in the particular bore. This would be the end gap of the ring, as installed on the piston when the whole thing is installed in the cylinder bore. Too much of a gap, especially if the end gaps are not staggered in their positions on the piston, might be some of where the compression is going, but not necessarily all of it. With the ring pushed down into the bore, evenly, you can then use a feeler gauge to check the end gap.

In order to make sure the ring is positioned squarely in the bore, the "tool" I've seen my machine shop associate use is basically a 3/8 drive socket with an Allen wrench section staked into it (as something you might use if you had Grade 8 Allen head bolts in the place of normal hex-head intake manifold bolts). Use the Allen wrench section to push the ring down, using where it is inserted into the socket as the "stop". Use 4 or more "pushes", evenly spaced around the circumference of the cylinder bore, to get the ring into position and to make sure that everything is measured at the same point in the cylinder bore.

The other tool that can be used is a "dial bore gauge". This is like a dial indicator with a handle on it, which will allow it to be placed diagonally in the cylinder bore to check the size and to check for changes of diameter in the area where the piston/rings contact it (the wear portion of the cylinder bore). When properly calibrated, it can reveal the true condition/shape of the particular cylinder bore. Then you can use a large micrometer to measure the piston diameter (according to the method used in the Buick shop manual). This should give you the exact piston clearance in the bore.

In prior times, the method was to use a bare piston (without the rings installed) and place it in the cylinder bore. Then some long feeler gauges of brass would be used to check the clearance between the piston and the block. This can be somewhat time consuming to do, but that's the way it was done in prior times, when measuring instruments as micrometers were "high tech" and expensive. Kind of like using some flat feeler gauge stock to measure the gap in a set of ignition points.

Considering that you have many concerns about the condition and such of the motor, it might be best to find a competent machine shop that is well-versed in older engines. All engines might go together with nuts and bolts, but knowing how to get everything "right" is a crucial issue (as Old-Tank revealed a while back in assisting in getting 5563s '55 Special V-8 rebuilt). Hopefully, there will be some (with all due respect!) older machinist/engine builder that was around when it was common to rebuild those motors in the shop and would take the time to recall/research how to do everything as it needs to be done. Possibly there might be some other BCA members in your area with similar engines that might provide some guidance in this area? This is one time that you'll need to be an "informed consumer" and ask questions and listen (and pay attention to body language) to how the questions are answered.

Just some thoughts,

NTX5467

[Only Buicks]

More Data and Findings from 1938 - 248 cui s/n 43463129

This morning I started taking measurements and comparing things to the shop manual and have some additional info. First I measured the ring gap. I assume this is the gap left in the split of the ring when it is in the cylinder? I removed the top ring from the piston and placed it in the cylinder. Using a carpenters square, I set it at 1.5" and used the top of the block as a square surface. I then got the ring in the cylinder and checked for squareness. I measured the gap at 0.035 - 0.036 with feeler gauges. I then set the square at 4.0" and moved the ring down into the cylinder, got it square and measured the gap again. This time I came up with 0.032 - 0.033 gap. I then moved it back up to 1.5" and rechecked and came up with the same measurement as I had previously. The shop manual says this gap should be 0.010 - 0.015, I guess this is not good news.

Next I measured the width of the rings with dial calipers and compared them to the shop manual.

Top Ring = 0.092, next = 0.098, next = 0.192 and last is 0.182. The shop manual says the top should be 0.125, next should be 0.09375, and the next two should = 0.1875, more disturbing news.

Next I compared the picutres of the pistons in the shop manual to what I have, my conclusion is the pistons that are in the engine are not 38 pistons, they actully look more like the pistons that where in my 1941 - 56c except the description in the 41 shop manual says the two lower rings are of the same type which is not my case. The third ring down is a two piece construction where the inner portion has 6 or 8 lobes around the circumfrence, then the outer ring mounts over it. The 38 shop manual says there are oil holes in the piston to allow oil to drain thru the skirt, again not the case with the pistons I have, more disturbing news.

As close as I can determine using dial calipers the piston measures 3.097 and the cylinder at the very top measures 3.106, not sure what kind of news this is!

Now, I do not remember if I shared the following info. yet or not. Currently the car has an engine in it that I picked up in Toronto from a 1938 McLaughlin s/n 3292378. This engine runs, has good compression (130 - 135 lbs.), vacuum test looks ok except every now and then the engine has a slight skip. This shows on the vacuum gage as a slight dip, about 2 lbs then back up to 20 lbs. The skip is not cyclical, just every now and then. The reason I started playing with the original engine was that the Toronto engine is blowing blue smoke which looks like hell. I thought this might be a valve problem so I removed the valve cover to check the lash. This is when I noticed that the valve keeper on the #1 exhaust valve had been mickey moused. Thats when I said to myself, why screw around, lets check out the original engine and swap them. Now I am stuck as I am not sure which engine should be in the car. I thought maybe I should just swap the heads between the two (Is this a good or bad practice ?) then I noticed that the casting numbers are not the same between the two. The original engine has a casting number of 1302277-2 located on the top center of the head under the valve cover. The Toronto engine has casting number 1289261 located just above the where the intake & exhaust manifold mount. Both heads have a B3 cast in the center of the head where the spark plug wells are. The original engine uses 14mm spark plugs where as the Toronto engine uses 18mm plugs.

Given all this information, which engine would you put your efforts into? I have ben accused of doing things backwards but this is getting rediculous.

Totally confused in Connecticut!

[Only Buicks]

pic of orginal head casting

[Only Buicks]

looking down into cylinder

[Only Buicks]

close up of valves on original engine

[Only Buicks]

pic of exhaust valve keeper for #1 cylinder on Toronto engine

[Pete Phillips]

The compression ratio of a 1953 engine would be around 7:1 or 8:1; whereas in the 1930s, compression ratios were on the order of 4.5:1 or 5.5:1, so a figure of 60 psi on the '38 engine, being about 50-60% of the 1953 figures, would be about right. A 1938 shop manual will tell for sure what the compression test results should be.

Pete Phillips

[Guest ZondaC12]

i recall from my shop manual 1938 for the 248 ci the figure mentioned is exactly this: "110 to 118 psi".

the compression ratio is 6.1:1

[Only Buicks]

NTX5467,

Would love to get your opinion on my recent response to your post.

Thanks for all your input so far.

[NTX5467]

Glenn, I did a post last night and it did not make it, for some reason. I'll recollect my thought . . .

With the piston to cylinder wall clearance, it might have run but would have had piston noise. Possibly several reasons that the engine was cleaned up and the old parts were used, which would have included: what the owner could pay for, what the machine shop could OR would do, and parts availability.

Your manner of checking the rings end gap is good. The larger gap could be where some of the compression was going, I suspect.

On the underside of the pistons, there should be some sort of casting number (which could look like a GM part number, but not be a part number as such that you would find in the Buick parts book). Many of the older paper catalogs from manufacturers as Silvolite would have listed their pistons that would match particular GM piston casting numbers. This number could be on the underside of the dome or inside of the skirt.

I found my "speed screts" book, 1954 edition. In the Buick Straight 8 section, it talked about how those Buick engines would usually go 100,000 miles without having to have the cylinder head removed--when other engines would do good to go 1/2 that without needing a valve job. It also mentioned a difference in the pistons from you year and 1941--something about the Turbulator being recessed on the early ones and flatter on the later ones.

I went into the Egge Machine website and looked around a little bit ago. They show the same piston for the Buick 248 from 1938-1949 (in a .020" oversize, the part number was E105-20). It appears, according to them, that one replacement piston would cover all of the Buick 248 Straight 8 engines. In this day and time, compared to 40 years ago, that could well be the case. How they would all work and at what compression ratio would probably be due to the combustion chamber volume of the cylinders they'd be working with.

Typically, casting number will follow model years (later years have higher numbers). As the lower number cylinder head came from the same year McLauglin Buick, that could be an interesting situation to reconcile--unless you had a parts book to look at, which might also have casting numbers for particular replacement part cylinder heads AND their application.

In the speed screts book, it advocated using 1949 248 connecting rods as they were "inserted". Sounds like they would retrofit for all earlier 248s. There was also caution about when you put the rod back onto the piston pin and torqued the bolt on the pin-end of the rod to keep everything together.

I think that at this point in time, it might be wise to seek out an old auto supply that would have the earlier edition parts catalogs that would have information on the 1938 Buick motors in them (casting numbers, etc.). Using that information, hopefully you can determine just what assemblage of parts you do have.

Please keep us posted as this progresses. I believe that from what you've shown us so far, the whole situation will end up at a competent machine shop that knows about those old Straight 8 Buick engines and is willing to do a great job with the work that they do.

Just some thoughts,

NTX5467

[budDynabuick]

Glenn, I did a post last night and it did not make it,

Yes NTX this happens to me as well so i started putting the post in note-pad and then cut & paste so it doesn't time out since i am to SLOOOOOOOOW.

hey Glenn. Your vac reading would indicate a sticking valve which could effect your compression reading I think. BTW, I love the straight 8!!!

Keith

[budDynabuick]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Glenn, I did a post last night and it did not make it,

Yes NTX this happens to me as well so i started putting the post in note-pad and then cut & paste so it doesn't time out since i am to SLOOOOOOOOW.

hey Glenn. Your vac reading would indicate a sticking valve which could effect your compression reading I think. BTW, I love the straight 8!!!

Keith </div></div>

Hey Glenn. Have you driven the car under a load? How far? With your vac reading indicating a sticking Valve/valves you may just need to drive it to loosen/free- up those valves. Just a thought.

keith

[Only Buicks]

Hello all,

First I would like to Thank you all for your help and input. This past weekend I pulled the head off of the engine that is in the car (McLaughlin engine) and swapped the head from the original engine. Just as a side note, when I pulled the head off every piston had about a teaspoon full of oil sitting on it! Anyway, I swapped the head and whala, the smoking problem is gone! I don't know how that much oil could be making its way down the valve stems but at that rate this baby would have consumed as much oil as it does gas.

Thanks again for all your replies.

Happy Holidays and Merry Christmas to all.

<img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" />

[Dandy Dave]

I have rebuilt a fair amount of engines myself and this reminds me of a tractor I help a buddy with years ago. When we pulled the head off it was just as you said," about 1/2 inch of oil on each cylinder. You could put your finger on the piston and easily wiggle it an eighth of an inch. I'm still not sure to this day how this thing ran, but it did. It could have been all the excess oil on top of the pistons raising the compression??? Or thick oil. It was not beyond the cheap old farmers around these parts to put 90 wt in the crankcase just to get the last few nickles out of it. The rings still had enough tension and could be seen between the cylinder wall and piston. My theory was that there must have a vacume caused somewhere in stroke cycles. Most likley the intake stroke coupled with bad valves and or cam caused the oil to be sucked into the piston tops. The rings, desined to hold compression in the chamber, held the oil up there. One thing I could never figgure out is how it did not instantly drown and foul the plugs???

From looking at the photo that you posted of the piston in the cylinder it is clear that the mesurement of 3.100 (Read, three inches one hundred thousands and is close to 3/32cds of an inch) has to be a typo. Ten over on a three inch bore would read 3.010 (Read, three inches ten thousands.) It is clear that the pistons are standard as it is marked at the top. With most engines the pistons are made to fit the bore. ( there are several exceptions out there.) In other words, the pistons are made smaller than the bore and made with enough clearence for expansion as things warm up. If the pistons are meant to fit a 3 inch bore, then, they are already under size by 5 to 10 thousands or more to begin with. If your cylinders are 10 over, it could very well be beyond exceptable wear limits for your application. The rings need to have nice sharp flat edges. It there is excessive piston slop the rings will wear with a radious alowing compression to slip by. Doing a leak down on the cyliners is a good idea and is commonly done on aircraft engines. As far as honing goes it provides two nessasary functions. First and formost it breaks the glaze off of the cylinder so the new rings can seat, and also as said before, small receses for the oil to sit in. If you look in a vintage motor manual, or servicemens book for your car, the information on exceptable wear in the parts like piston skirt to cylinder, and allowable ring gap will be there. The best thing that you can do is invest in a book if you do not have one. The information that it will provide will be invaluble now as well as in the future. Happy Trails, Dave!