After talking to Mark R. (cutlassefi) about buying some go fast items from him, I decided to check a couple other pistons in my 68 Cutlass 350 to try to see if the engine has been rebuilt at some point in the past. The engine was originally a 9 to 1 2bbl. The odometer shows 49K, and from the overall condition of the car, I think it is 149K and not more. The engine does not appear to burn any oil, and the compression is 135-150. I suspect that the engine has been rebuilt at some point in time since it runs really well and since it has been repainted red, implying that it has been out of the car for some reason.

The good news is that the dish looks very much like the 9 to 1 pistons from a 68 350, and I finally found a half moon notch using a borescope (see the first photo below.) There appears to be some numbers and / or letters on the face of the piston, but I cannot make it out. Did factory pistons have numbers or letters on the face? Can anyone tell if these are the factory pistons or one of the "shorter" aftermarket cast pistons?



Half moon

Numbers and letters?

Dish

Numbers and letters?

Thanks in advance!

 

The factory pistons will have letters stamped on them with a corresponding letter stamped on the block deck next to each cylinder. Oldsmobile semi-blueprinted the engine at the assembly plant by measuring each cylinder bore and selecting a piston sized to fit each bore. This is clearly explained in the chassis service manual.
For example, the 350 cid pistons were separated into 4 sizing groups:
Piston "A" were 4.05525-4.05475 and fit bores 4.0560-4.0565
Piston "B" were 4.05575-4.05525 and fit bores 4.0565-4.0570
Piston "C" were 4.05625-4.05575 and fit bores 4.0570-4.0575
Piston "D" were 4.05675-4.05625 and fit bores 4.0575-4.0580

See pics below. These were from a 1976 350 engine.

Rodney

 

Thanks, cdrod. I certainly don't see anything as obvious at what you are showing. There may not even be letters or numbers there. I may just be imagining them in the corrosion.

 

It's likely that the cylinders were over-bored if the engine has been rebuilt. In this case you could see 0.030 or 0.060 stamped on the tops of the pistons to indicate the oversized pistons, but I don't see evidence of that in the pics you've posted.

 

Stolen from an old Joe P. post

 

Thanks. I do not see evidence of this either. To me, with the half moon, they really look like the factory pistons, but they do appear to have some number lightly etched into the tops. I have not seen any photos of factory pistons like that.

 

Thank you. I looked at that image as a reference, too, and the half moon shape looks exactly as it should for the 9 to 1.

Does anyone know if any aftermarket cast pistons used the same half moon shape?

 

Cant say I have ever seen an aftermarket piston with that type of notch for the front mark. Personally I would bet it is a stock piston. The only way to know for sure is to pull one out and look at it

 

LOL

Yes, you are correct. At some point, I will pull off the head and find out for sure what has been done.

I agree, that it looks more and more like it is the factory piston which would be great. Since the engine doesn't burn oil and now runs like a top with the new intake, 4bbl and MSD Ready to Run, I am planning to put a bigger cam in it along with the new Edelbrock small block heads, a Performer RPM and a bigger 4bbl whenever the new heads become available.

If the pistons turn out to be the "shorter" cast replacements, thereby reducing my compression well below 9 to 1, I might just rebuild the engine instead to go with flat tops. That is a lot more time and money, though.

 

Oldsmobile V8's were very durable for their time. We got over 300,000 miles out of the 350 Rocket in our 75 Cutlass, really abused at the end. We sold it for scrap with a seized brake caliper and knocking engine. The wrecker sold it and it was on the road for another 5 or so years! Our 81 Delta 88 with 307 went over 420,000 km, the motor over 450,000 before the timing chain skipped. Still had Ok oil pressure and used almost none. That had 3000 km oil changes for the first how many years. A stack of regular service bills from its purchase in 1982. My point is I choose Oldsmobile because they were so dam durable. I didn't pick my cars off the Internet because I thought they were pretty. The Olds 307 and 350 especially were better built motors from anything else from that time. Dodge and Fords especially were horrible cars in comparison. Even the sbc almost always flattened cams around 100,000 miles till the early 80's. I got 5+ years out of a $120 76 Olds 350, told 90,000 km from a 76 Delta 88. 140 to 142 psi compression with Performer cam, 150 with the 214/214 cam and #6 heads, 9 to 1 vs 8 to 1. It had 20 psi hot in gear, 25 psi in park and 45 to 50 hot cruise oil pressure with 5W30 oil! My 76 motor had no oversize stamping anywhere and had the steel shim head gaskets and a nylon coated cam gear in surprisingly good shape. If you find these items, it is probably still original. Spend the extra money and get the .027" MLS to improve quench with the new heads. I plan on going to a .042" MLS as my pistons are .003" above deck. For now with the #6 the Felpro Blue measure a whopping .042" compressed FYI, perfect for my build and iron heads going on for now. The info on the internet shows .039" compressed. Cutlassefi told me .042", I put a caliper on a used set, .042" is correct. It may be original, even being 50 years old.

 

That is good advice. Thanks!

I can imagine a number of reasons why someone might pull the engine and repaint it without doing a full rebuild. Maybe the timing chain need to be replaced (that would be nice), or maybe the oil pump and oil pan gasket needed to be replaced. Who knows?

The other encouraging sign is that I can see the original gold paint on the block where the red paint is flaking off. I would assume that if the engine was bored .030 over, the original paint would have been lost during the process. So between the gold paint beneath the red paint and the half moon shape on the pistons, I am guessing that the pistons are stock.

Is it possible to check the timing chain with a borescope down the oil fill tube, or would I need to remove the fuel pump and use the scope there?

 

You may be able to, if you can get the angle right. I have a 73 motor painted corporate blue, very hard to remove. It still has the original pistons in it. But had Felpro Blue head gaskets and a double roller timing set along with a new rear main shows at least gaskets were done. I found about one spec of factory Olds gold on it, they were very thorough. Yes, my block barely had any paint on it after getting machined.

 

Thank you, sir. I tried getting the borescope past the bend at the bottom of the oil fill tube, but I could never get the angle right.

I think I am satisfied now. As soon as the Edelbrock heads are available, I will go ahead and order the heads along with a hotter cam, Performer RPM and all of the necessary associated parts (new timing chain, lifters, etc.) I'll pull the engine to overhaul the top end and take the opportunity to clean up the engine bay.

Thank you all for your help.

 

Keep in mind that there is a metal plate that is there specifically to prevent any objects that may fall into the oil fill tube from getting into the engine. It would be impossible to get your camera past that.

 

Good point, I forgot about that bad boy. Looks like the fuel pump does need to come out. It is possible just the timing set and gaskets were done like my 73 motor.