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1966 300cid engine / ST300 trans work questions


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Dear Buick enthusiasts,

I am Stefan from Germany.

As some of you might remember, I own a 1966 Skylark Convertible with 300 cid V8 engine and Super Turbine 300 2-speed automatic transmission.

My father-in-law bought that car new in 1966.

The engine/gear box has never been out of the car. The only major repair was in 1985 when I replaced the camshaft and lifters.

Although the car was still running reasonably well, it has developed some faults over the years:

  • Some significant oil leaks engine and trans
  • Oil consumption is about 1 quart per 1,000 mls
  • exhaust definitely smells burnt oil
  • can not adjust carburetor correctly/ assume false air on intake manifold
  • pinging under heavy load although timing is on the dot

What I have done/checked so far:

  • installed reman carburetor
  • installed reman distributor
  • replaced spark plugs and -leads
  • checked compression, all within tolerances
  • oil pressure is good, no knocking noises from bottom
  • lifters are quiet
  • coolant was bright green, no scale or rust.

Yesterday I pulled engine and trans. All went fine, no problems at all.

Here's what I'm planning to do:


Remove intake an heads. Remove valves from head, de-carbonize head and valves.

Check valve stems for play, replace valve stem seals.

Inspect cylinders bores for wear.

Replace timing gear.

Repaint engine to original spec.

Put everything together with new gaskets.


Replace extension sleeve due to excessive play of propshaft and corresponding leak

Replace speedo drive seals due to leakage

Replace oil strainer


Would you take apart the engine bottom end of the engine?

Would you pull the pistons to check for problems/wear?

I'm buying most of the stuff from Rockauto, since they offer export services and deliver rather fast.

Are there any brands of material I should stay away from?

Should I pull the torque converter from the trans to drain the oil completeley? Trans oil was ok, not brown or burnt. I know how to install a torque converter correctly.

Should I use a primer before repainting engine surfaces?

I should mention that I have the 1966 factory Chassis Service Manual as well as the corresponding Fisher Body Manual.

Please let me know your thoughts.

Thanks in advance and best regards


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I'm sure there will be someone more qualified than me to offer you the advice you need but the question that comes to mind for me is how many miles has the car traveled?

If it's high mile car you may be taking a risk only working on the top end.

If it's a low mile car then a tidy up with new gaskets may be fine.

While the engine is out I'd have it measured and at least go with new rings and bearings if the bore is OK.

How about some Photos...........we like photos:)

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Hi Brian,

thanks for your reply!

Stupid me, I forgot; we're now at 237,000 kms, that is some 147,000 mls.

I'm a bit afraid of touching the lower end, cause it's working fine and some friends of mine have had bad experiences with foreign (on my side of the pond, that is) material being overhauled by local machine shops.

Here's a photo of my car.


The hubcaps are not correct for the specification, but I have the original ones (spinner type) as well.

Problem is they tend to come loose and fall off.



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The 300 is a very stout engine. Typically you should get 200,000 miles from a well cared for engine. I am mostly familiar with the '64-65 300s but there isn't much change in '66. The main fault with these engines is the timing chain and gears. Buick coated the camshaft gear with a phenolic (plastic) coating for noise which breaks down around 100-120K miles. I know the '65 300 had a hot air choke with a stove pipe from the right manifold. These tended to carbon up especially in the passages under the carburetor. I am assuming your engine is a 2 barrel as in '66 if you wanted a small block V8 4 barrel you got a 340. I recommend you have the intake manifold boiled out as well. I never could get an aftermarket carb to perform as well as a properly rebuilt factory carb.

I would recommend leaving the bottom end alone in your case. I assume that the car isn't driven a lot of miles and if it isn't giving you problems you should get many more miles from it. That said, if you have pulled the engine, there wouldn't be any harm in pulling a rod cap or 2 and checking out the bearings. If your oil leak is a rear seal, then you might as well rebuild the bottom end as long as you are in there.

As long as you have the transmission out, pulling and draining the torque converter would be a good idea. As far as primer, Buick didn't use primer on their engines. Just use a good quality high heat engine paint.

Good luck and that is a pretty car!

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ol' yeller,

thanks for your advice. I will replace the timing gear for sure.

The original carb choke system has bitten the dust decades ago. The pipes were rotten, the heat riser broken and the bi-metallic-spring defective.

That was back in the eighties when it was was nearly impossible to get spare parts for this car over here. So I converted it to an electric choke system taken off a Camaro...

You are right, the carb is a 2 bbl.

The rear seal is not leaking, I would prefer to leave that alone if possible.

Thanks for the hint regarding painting, there's an US-car store around who stocks engine paints matching the original colors.

BTW, how is the cover called, which is located between engine and trans covering the torque converter? It has been lost on my car. Ideas anyone where I could get one of these?

Kindest regards from Germany


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I think you are asking about the flywheel cover or sometimes it is called the flywheel inspection cover. I may have a friend who has one of these. He is also very much into the '64-65 Skylarks. He is reliable,reasonable and I'm pretty sure he will ship overseas as he does a lot of business on Ebay. I'll check with him and then PM you his email so you two can work out the details.

My experience is more with the 4 barrel carbs. I think the issue is not so much related to the stove pipe but more to plugged passages under the carb which warms the carb upon start-up. They can also overheat the carb if the proper plates and gaskets aren't used. I'm not sure that the 2 barrel carbs have the same setup. Used carbs are usually very corroded on the bottom plate because of improper sealing which makes proper sealing even harder.

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Ol' yeller,

that would be great! I have an Ebay account with 500+ positive feedback, so if he likes to put it on Ebay I would be happy to buy there. Please let me know.

You are right, these heat passages were blocked in the intake, but the stove pipes were rusted out beyond repair too.

In case I could get all necessary hardware (i.e pipes, bimetallic element at carb, heat riser etc), I'd be happy to convert it back to the original setup.

Thanks again, yours


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Hi all,

first of all thanks again to ol' yeller, who got me in touch with a friend of his. He sold me a flywheel cover which is on the way right now.

I have now dismantled the eingine and found a fair amount of sludge in the oil pan, but no metal or such.

I've got two questions for the experts:

While the left cylinder head is rather clean, the right cylinder head is full of crud. See attached pictures. What is the reason for this?



Since years I could not adjust the timing by the timing mark on the timing cover/harmonic balancer.

Now it's obvious why. Please have a look at the picture, how can that be?

The harmonic balancer rest on a key on the crankshaft, so it can't be misplaced.

How would you rectify this?


Thanks in advance for your input,


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Stefan, When you pull your timing chain cover, I'd bet that the timing marks on the gears are not lined up properly. Both the camshaft gear and the crankshaft gear have marks that need to be aligned when installed. I'm surprised that it did run as this would mess up your timing big time.

As for the sludge it is probably because of the intake passages being plugged up with carbon. The right side is also where the PCV valve is located so I'd also make sure that is all functioning correctly.

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The timing chain has nothing to do with the piston position in relation to the crankshaft or balancer; only the valve position in relation to the piston. The balancer is probably defective. The cleaner head was probably removed for service more recently than the other.


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Willie & Greg,

thanks for your replies. I agree that the harmonic balancer is defective, I just can't understand why. The balancing weight and the disc which carries the timing mark are riveted to the keyed base with six big rivets. Those are all undisturbed.

How in hell can the disc rotate relative to the base while the rives are undisturbed?

In case the balancing weight has still the correct position relative to the base, I'd be tempted to put a new TDC mark on the disc and be done with it.

OTOH, if it drifted off once, it may continue to do so and be incorrect in the future.

I think I'll look out for a new/used one.



EDIT: Searched the net and it seems not an uncommon problem that the timing marks do slip...

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A few thoughts of mine: First do a compression check on the engine. This will tell you alot of what is going on. Now if the timing chain has skipped the compression test will be useless. Perhaps someone put the wrong balancer on the engine???

My feeling is that at that mileage the engine is probably in need of a TOTAL rebuild. Doing the heads and leaving the pistons/rings alone will only prolong the agony. You seal up the top, the bottom then goes. Seals, well that is automatic if the engine and trans are coming out. Rear main seal - replace, front pump seal on transmission - replace, transmission external seals such as tailshaft, accumulator cover, etc. -REPLACE.

As the mechanics say: You can pay now, or pay more later.

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thanks for your thoughts. I performed a compression check last year, and the results were satisfactory, except one cylinder, with was just under the specs.

In the meantime, I have completeley dismantled the engine. Results so far:

- Cylinder wall wear within limits, no unusual scores or scratches

- Cylinder bore size within limits

- Piston ring gaps above wear limits, will fit new rings standard size

- Pistons with normal wear signs, nothing unusual, can be reused

- Connecting rod jounals fine and within specs, no need to regrind or overhaul

- Connecting rod bearings at top of wear limit at .004 - 006 clearance, but no scratches or burrs

- Crankshaft main bearings good and not worn to a grade that a regrind should be done

- Crankshaft main bearing clearance also at top of limits at around 0.005

- Crankshaft end float good

I conclude to replace piston rings, crankshaft main bearings and connection rod bearings with standard size new parts.

Since I'm going to buy complete gasket sets for engine and transmission, replacement of the seals and gaskets no question.

Now for parts ordering...

Thanks all for your assistance


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you are definiteley right, it's a fine piece of machinery.

The flywheel inspection cover has arrived yesterday, thanks a bunch for your help. It's great to have that long missing part!

I'm in the process of cleaning the engine inside and out.

All the necessary parts are ordered, I hope they will arrive soon and fit well.

I'll let you know how work progresses, will take some time thou.



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  • 3 months later...

Dear Friends,

it’s been a while that I wrote about my 300 engine overhaul.

First of all let me thank again for your advice and encouragement.

Now here’s about what happened in the meantime.

After taking it all apart I found the engine to be in not too bad a shape.

The upper edge of the bores showed a significant ridge, but no scoring or untypical marks. The crank journal surfaces did look fine as well and the bearing clearances were within specs.

So I decided to re-ring the pistons and go with standard size new crank main and con-rod bearings.

I got all parts (and some more) from Classic Buicks. All was delivered in time, no problems.

While waiting for the parts, I bought a ridge-reamer to cut the ridge off the bores. When it arrived, I tried to use it but I was scared to scrape off more the necessary and destroy the bores. So I used another tool to hone the bore surfaces, one of these three-legged-hone-tools which you fit to your drilling machine, don’t know how it’s called in English. I used it a bit more intensive at the top to even out the ridge, it came out pretty well, at least optically. :-)

After arrival of the parts, I fitted new rings to the original pistons, installed new bearings and put the engine back together with all new gaskets.

Without further testing, I installed the engine and trans in the car.

When first started, it ran pretty rough but evened out except running on seven cylinders.

Checked compression and found cyls #1 to 7 in good order and #8 zero.

Removed intake and valve cover and could not find anything wrong.

Major disappointment.

After some fiddeling I decided to bite the bullet and remove the engine again for further examination.

Went much faster this time!

Stripped the engine again, removed the #8 piston to find – nothing.

Being rather puzzled I decided to do it right this time.

Brought the complete short block to a local machine shop to have it measured.

With the results I ordered oversize bearings and oversize pistons (again from Classic Buicks) and had the block re-bored and honed and the crankshaft ground.

I had the machine shop reassemble the short block to avoid liability questions in case of problems.

After the engine being mostly assembled on the stand, I decided to do a compression check with the engine still out of the car.

Guess what, still no compression on #8. Darn.

The intake manifold and valve covers were still off, so I could see the valve mechanism working properly. But when I backed off and re-tightened the rocker shaft in question I could see on very close examination, that the #8 exhaust valve was pushed down by a very tiny amount, being not fully seated! No wonder there’s no compression!

I found that #8 exhaust lifter was locked hydraulically. Of course the method of choice would be to renew at least this lifter, but since this would have meant another delay for weeks (remember, I’m in Germany) I took it apart, cleaned it and put it back together.

Compression now fine on all Cylinders!

Reassembled rest of engine, put back in, started right up! Had some lifter clicking for the first minutes, but that disappeared soon.

The first test drives showed that the engine performs well and runs smooth.

It still stumbles quite a bit when I start from a standstill, will tweak the carb adjustment the next days.


  • I don’t know if the “cheap” overhaul with just new rings and bearing shells would have been a success. The compression of #1 - 7 was good. So it could have worked
  • On the other hand the bottom engine is now perfect for the rest of my life (fingers crossed).

With kindest regards from Germany


A picture of the nearly finished engine:


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Those old 300's are very hardy motors. They are almost impossible to kill. I should know because I did actually destroy one back in the '70s. You should be set for many years of happy motoring. Congratulations.


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Guest my3buicks
Those old 300's are very hardy motors. They are almost impossible to kill. I should know because I did actually destroy one back in the '70s. You should be set for many years of happy motoring. Congratulations.


Fantastic engines for sure!! Surprisingly quick, smooth and responsive.

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