Festus

Original 350 in my 72 Cutlass needs some work. Pulled it out to reseal and fix leaking head gasket. Measured crank and cylinders and ordered standard rings and bearings to freshen it up. It is a 108,000 mile engine. Then while cleaning the pistons, I dropped one and broke it. For good measure, I dropped and broke one more. Since I need two pistons, I would like to increase the compression from the factory 8.5 to 1. I have pieced together many super low buck engine combos in my day. How stupid is it to find 8 higher compression standard pistons, measure each piston and bore and install them in the holes with the best clearance? With the Olds compession heighth and stock 7a heads, what true ratio would flattop pistons yield? Any ideas other than those I mentioned? I will drive the car less than 3000 miles a year and will overhaul it right when finances allow. I would just love to get it drivable for the summer for little cash. Call me cheap but times are hard right now.
Thanks,
Festus

oddball

How's the ring ridge?
I would assume there'd be about an extra 1thou of clearance, so you'll get quite a bit of clatter. If you dingle-berry hone and file fit oversized rings then it'll probably run. For a while. It'd be noisy and would be hell on the skirts.
If you can find forged pistons, then that'd probably take care of the clearance issue. But not talking super-low-buck anymore.

My true compression with the supper-small dish (1cc?) was about 9.5:1. They were 10.25:1 at the factory, but that was with the shim head gaskets. So note that it'll fall to ~8:1 with the stock pistons and the typical Fel Pro head gasket.

MDchanic

In theory, 7A's are 64cc heads, the same as 6's and 5's, so the same compression ratio details should apply (with OEM 0.016" shim head gaskets - newer 0.046" FelPros will drop your compression about 0.5:1): 14cc gives about 9:1CR, 6cc gives about 10.25:1, and flat gives about 10.5:1.

Since actual CC volumes may vary, as may deck heights, piston volumes, and head gasket thicknesses, you should measure everything and calculate the CR of the actual setup you'll be using.

As Oddball said, watch that ridge, and use a ridge reamer carefully even if the ridge seems insignificant.

As far as I'm concerned, you should be able to get away with a set of used pistons just fine, so long as you're careful and clean - these are basically tractor engines, and will put up with a huge amount of abuse, especially if you keep the revs below 4,500 (as a stock auto transmission will do). CopperCutlass races an engine with used components and has had no problems.

- Eric

Rickman48

And if that 350's been overheated, you could be throwing your money away!!
Look around for a 68-70 motor - could be cheap!!

Festus

The ridge was very minimal. I have cut it out. To the best of my knowledge, the engine has not been hot. I have had the block and heads magnafluxed and professionally cleaned. They check out ok. Heads and block are also both flat. If I wait to do a full rebuild, the car will have to sit idle for a year or more. The engine is original and I would like to use it if I can. The crank is within spec and would receive a slight polish before reuse.

Festus

coppercutlass

As eric mentioned i do race with used components and i spin my engine to 6k rpm. I have had no issues other than a little blow by nothing bad just spent gasses going past the rings . I also had what i though was a bouncy oil psi turns out my valve lash was a bit off and could have possibly made my oil psi bouncy not by much though., a bouncy idle could have had something to do with it too its got a big cam . If you do use used pistons wheigh them wheigh the rods and make sure they are all with in factory low and high limits which is what i did. My shortblock consist of a 72 block and crank with a combination of 72 and 70 rods with cast flat tops from a w31 engine . The combination of rods is a long story some where bent to say the least . Btw i have some 23 cc dished pistons on the rods off a 72 350 that you can have just pay for shipping that way you can keep that car on the road untill finances allow flat tops

380 Racer

That is just plain wrong.

coppercutlass

In literature it's what's stated he did mention there are variables. I have seen small block heads that are closer to 64 cc and i had a set of 6's that where almost 69cc's . Granted #5's are generally what everyone wants because no egr bumps they tend to flow better so everyone say's they are the best etc. etc. An explination to why he is wrong whould be much more helpful than just saying some one is wrong imo. Cruising the boards to just tell people they are wrong with no reason to explain is kind of a slap in the face.

MDchanic

I'm happy to admit to being wrong if I am.

Perhaps I was caught up by the infamous "FAQ Misinformation," as I got the info there.

The FAQ says:
Please let us all know what the right numbers are, if you can.

Thanks,

- Eric

cutlassefi

I've had a half dozen 5, 6, and 7's come past me in the last couple of years. None were 64, I'd say 66-68 would be much more accurate.
My 7's are 66 with .025 taken off and larger valves installed with minimal sinking of the valve.

coppercutlass

That sounds just like the 6 heads i have.

MDchanic

Jeez.

I didn't say they were exactly 64cc.

I said,The point was to let him know that any of those should offer about the same compression for a quick and dirty stock rebuild.

I don't think any of that was incorrect.

- Eric

Festus

coppercutlass, thanks for the offer. A buddy called tonight. He found a running 350 for $300. Still in a licensed car so I can drive it. The owner built a 425 and says the 350 runs out very well. He'll pull it as part of the deal. I am going to look at it Wednesday. I can stick it in and drive the car while I save to rebuild mine. If I don't buy it, I would very much like to get two pistons from you.

Festus

coppercutlass

I have em sitting in the shop. I'll even clean the up for ya just pay for shipping. Im going there tommorow i'll double check to make sure i have two it's been a while since i looked at what i had but i should have 2

cutlassefi

That's fine, it's just that some on here insist that they're 64 because the book says so.
Point taken Eric.

380 Racer

I was posting from personal experiences. I had two pair of #7s and one pair of #6s, all virgin heads and cc'd at 68-69 ccs. The pair of 7s I used had a 62 cc chamber but were milled .045.

MDchanic

Thanks, Mark!

380 Racer

I'm the one that should be saying JEEZ. The way I read it was: the 7a have 64cc chambers.....just like the #6s & #5s. Let's talk theory, I have read that the main difference between my 6s (off my 310 HP 350) and w-31 6s was the valve sizes. Other sources claim they were blueprinted & maxed out from the factory. I'm sorry I couldn't tell you on that one.