opinions on a tapping noise

[Guest ZondaC12]

Hi everyone!

mid to late august i put the '38 special away in a garage im renting at a storage business for the winter. so no testing possible. but over the several months this spring and summer that i drove the car (put about 500 miles on it) after getting it running after 25 years of sitting (for those that dont remember), i noticed the following:

sitting in place warmed up, i could give the throttle a stab, it would rev up quick and smooth, with no tapping/knocking noises.

cruising, expecially at higher speeds (40 all the way to about 55, fastest ill go) is fine too.

however, at lower speeds, like about 30 mph, shifitng into 3rd gear and the engine at a lower speed, about 1500 rpm accelerating up from that, with moderate throttle, there is tapping. only under higher load and throttle though. i was warned by someone in my other thread once ("Continuation of my 1938 special.....") that knocking from a bottom end bearing could sound like a valvetrain noise. i disregarded this because the top of the engine is about where my knees are, if you draw a line from my knees to the firewall, and id hear this tapping as i drove, and then looked in the engine bay and realized the top of the engine sat low, so what i feared could be knocking was probably indeed in the valve train, and it really sounded like when i had the valve clearance set wrong and there was too much space and the valvetrain was making more noise.

but what i dont understand is why would the valvetrain make more noise <span style="font-weight: bold">under load</span>? have i answered my own question and it indeed doesnt? i mean theres no extra pressure pushing the valves up under load and more throttle, at least not when the rockers are contacting the stems right? if i am wrong please correct me.

what would be involved in dropping the oil pan and checking the rod and main bearings? (should i check both or are one type more likely to have problems?) is there some crazy tool to use? (i always hear people say "yep theyre worn about 3 thousandths" and stuff like that, so you cant just grab the crank and try and shake it to find looseness right?) the individual i think mentioned "shimming it" or "removing a shim" what is this and how is it done?

i am worried that i possibly beat my engine half to death this summer if there are bad bearings. could i have messed up the crankshaft in some way? it drove fine and had plenty of power, and the conditions i described above at higher speed driving. i dont mind the noise, i figured it was just the valvetrain and it really gives it character of that period if thats the way they were supposed to sound, and in that case im fine with it. but if the sound means something is wrong then i cringe everytime i hear it and looking back it was there always under harder acceleration so im sitting here wondering how much wear and tear may have taken place and wondering why i didnt throw a rod or something already.

i want to plan this all out so next spring i can get going on whatever i have to even if its, i dunno replacing a bearing if it gets that extreme? since im in college now i get summer vacation from may 2nd to aug 28th next year so ill have 4 long months and a start in the middle of the spring so i can get going early and be able to have the thing reliable and running correctly for the summer.

Paul

1938 buick special

[Guest Oracle]

Piston slap can sound like a machine gun, and it can occur at specific speeds. You can "drive through" piston slap. In my limited experience tappets as we call them can cause top-end clattering if not set properly.

[Bill Stoneberg]

A stupid question here..... Are you sure what your hearing is valve/ tappit noise and not knock from having bad gas/ poor timing etc ? I would think that would be a possibility. How is your oil pressure ? Low oil pressure is usually a symptom of a worn bottom end.

As far as pulling the pan, it would be a major pain while the engine is in the car. You have to get undeerneath it and fight all the crossmembers. Not saying it couldn't be done.

To measure you would use a tool called Platicgauge. You take off a rod or bearing cap and then put a little piece in and put the cap back on and torque it down. Then you take it apart again and measure how much the platicgauge flattened out. Its made out of Plastic so it wont hurt bearings.

On your car to change the tolerances you add or remove shims between the cap and the rod or bearing. Do the above procedure Measure, add shims, repeat till correct.

[Guest ZondaC12]

i have been asked about the pressure before, and that is acually ok, until it gets real warm, its right at 30, then once warm, and especially really warm (after some highway driving) then at idle it will be below the marker between 0 and 30. but once you get going again at 30+ mph the pressure comes back up to about 30 (engine still hot obviously).

i actually had the pan off before i ever got it on the road. as opposed to my '87 cougar if i ever tried to, the buick's pan comes off like nothing, not ONE object/crossmember/anything in the way of dropping it. i originally dropped it to get the 1" of sludge off the bottom of the pan.

im 99% sure its mechanical, not "ping" from spark knock (pre-ignition right?) i have fooled around with that "octane selecor" and the one direction i rotated it was hard to crank (spark advanced too much) so i have it like halfway between and it runs good and starts easy, so i take that to mean the timing is right. besides again isnt it like a "ping" sound too, not the sound of a valve lifter? (because it sounds just like when my clearances were too big, not a metallic pinging noise. its like a "TOCK" sound).

also, when my valve cover is off at idle it sounds similar to yours in the video on your 1950 estate wagon site called "the engine runs again". not quite that loud, as you mentioned on the page, your valves werent adjusted yet, but the type of sound is the same.

[Mark Shaw]

Paul,

I may have been the one who suggested that a piston rod bearing may be making the tapping noise. I had exactly the same symptoms in my 31 Buick (the first straight 8), and it turned out to be a badly worn bearing on the #6 piston. I am about midway through fixing the crank bearing journal, and will be pouring a new babbit bearing this week end.

Since the straight 8 pisons are so small, they will not make the typical low knocking sound that bigger piston engines make with the same problem. I suggest you save yourself a lot of time, worry and potentially $$ by removing the pan and just wiggling each rod bearing. If you have been hearing a loud tapping sound, you will soon feel and hear which one(s) may be loose.

If you do have loose bearings, you should be able to remove shims (very thin flat metal pieces) from between the bearing cap and the rod. If you have no shims, it may mean that someone already removed them.

Mark Shaw

[Guest ZondaC12]

actually i went back and searched and... it was you!

so if i cant wiggle anything its probably okay? or does that just indicate that it is REALLY loose?

i do know what the bearing cap is, and i know there are two bolts on either side that hold it together. and in some box of my dad's in my basement i found these things that said "rod bearings" or maybe just "bearings" but there was a piston graphic on the side i think and they were these thin, half circle strips with a machined groove down the center. is that a shim?

i guess i could take the cap off. is there a certain torque spec on how tight the bolts should be? i saw a chart in this machine shop once that showed bearing wear patterns, so i know that if it looks black or really scraped away or scuffed thats a "spun" bearing or something, whatever it is i know its not good! am i correct in guessing that the whole bearing is two surfaces with a film of oil between them? that seems to be what i gather, but it didnt make sense at first because the only bearing i know is a ball bearing and seems engines dont use those? why not?

i know the last question is a bit of a tangent but thinking about it id like to know. this is one field of an engine i dont know a lot about. people say "he spun a bearing" or "the journals this and that..." (actually i do know what the journals are) "...four bolt mains..." and i feel like average joe because i dont know what it is! <img src="http://forums.aaca.org/images/graemlins/tongue.gif" alt="" />

[Mark Shaw]

Paul,

There are basically two types of rod bearings. Babbitt and insert bearings.

Babbitt bearings are made by pouring molten alloy into the bottom opening of the rod, letting it cool, and machining a hole in it that matches the dimension of the bearing journal(s). This is called line boring and is uasually done for all the rods at one time (in a line) after the crank has been machined (reground) to make all the bearing journals even, polished and the same size.

Insert bearings are C shaped "bearing liners" or bearing shell inserts that fit & lock into rods made specifically for this type of bearing. This more convenient design made insert bearings the modern way to make engines.

If you can't wiggle a rod bearing, it means that it is probably OK, or at least not badly worn. Measuring with plasti-gage is a more accurate way to tell. But you will be able to find badly worn bearings using the wiggle method. If you do find one or more that wiggle, remove shims to tighten them and then use plasti-gage to measure your clearance on each bearing.

Sorry, I don't have the rod cap torque requirements for your engine.

Mark Shaw

[Guest ZondaC12]

okay that clears things up a bit!

also i just wanted to know IF there was a torque spec. since there is, could i find it in the shop manual?

[Bill Stoneberg]

Yes.... I have noticed in my 40's manuals it is always at the front of the chapter.

[Guest]

Is your tapping all the time or only on deceleration? Later Buicks were notorious for wrist pin wear. Not dangerous but annoying. Sometimes it would return in as little as 20,000 miles after a rebuild.

[Guest ZondaC12]

no its on acceleration, not slowing down. slowing down or maitaining speed on flat ground with only as much throttle as is needed, its fine.