So I've torn the 350 down to the short block. No ridge on the cylinders which surprised me. I measured one bore at 4.059. Stock pistons in it. The only work the engine has ever had is a 'valve job' my dad had done on it in '78. The engine is in good shape. Factory cam still in it. I'm not looking to make a street monster. Just to up the power a bit. Already a 4bbl car so I'm going to dual exhaust and a bit more cam. The cam i'm thinking of is something like the Melling mc1287, crower 56260, or a comp cam xe250 or xe256.
Now for my conundrum. I know these are low compression engines. Since the cylinders and pistons are in good shape, do I just hone, replace the rings and throw them back in? Or, should I replace them to increase the compression. If I increase compression, which pistons?
thanks in advance for any advice..

 

My 350 I sent to the machine is what you described, no ridge and the cylinders looked good. All the cylinders would not clean up at 4.065". I went with the Mahle pistons in a 4.100" bore. What heads do you have? CP has a 2618 alloy 1.3cc flat top with a 1.5/1.5/3mm ring pack, that would work well with #8 heads. Cutlassefi worked with Mahle, a new coated 10cc dish 4032 alloy piston with a 1mm/1mm/2mm ring pack, also available in a 4.065" bore, would work well with early heads or the soon to be released new Edelbrock 68cc heads. A machine shop be able to tell you if you motor will clean up with a hone. Good luck.

 

Bore size is only part of the picture. What size are the pistons ? How much piston clearance do you have ? If you need new pistons, a higher compression piston won't hurt you if the deck and heads haven't been machined. Buy a name brand cam and lifters and degree the cam in.

 

My question is what year engine, compression ratio, and heads? Hard to give recommendations without real data.

 

It's a 1971 cutlass supreme. #7 heads. Like I said, original, bone stock with only a 'valve job' done in '78. I know I still need to look at piston to bore clearance and that the machine shop will be able to tell me more. This is not my first engine rebuild, but I'm seeing where the pistons for them are not like looking up pistons for other small blocks. I'm finding some vendors showing pistons for this engine and I can tell they are not olds pistons but chevy pistons. When they list the stock bore as 4.030, somethings up..
Olds 307 and 403.. those Mahle pistons sound like a good option! You wouldn't happen to know the part number..

 

Top pic should be what you are after. The rings are file fit FYI. Along with the new Scat and Molnar rods, these bring the Olds 350 to another level. You will be mid 9 to 1 compression, give or take. My block needed a .016" cut which will put the pistons .003" above deck. The Felpro head gaskets being .042" compressed thickness, .039" will be close to perfect.

 

ok, cool. i'll look those up. That's an option. I would be keeping the stock rods so it looks it would need to be hones a bit. thanks.

 

Even the Molnar rods needed the pins honed to fit. Those pistons are available in the 4.065", I mentioned and a 4.155" for a big overbore in 350 or to use in a 425.

 

Wow, just saw the price on those Mahle pistons. I see Summit has flat top speed pro's (L-2320f) but my concern is the compression may be too high and possible valve interference even though the cam i'm putting in isn't anything crazy. Guessing a chevy piston wouldn't work even if you bore since he pin size is wrong. Wonder why all these vendors keep listing them as an option?

 

The Chevy pistons have a different compression height as well. That’s the distance from the pin centerline to the top of the piston.

Your factory pistons have HUGE dishes in them. The 71-72 had largest dish of all the factory 350 pistons - a whopping 23 cc. You gotta get much smaller dishes in order to achieve an acceptable compression ratio.

 

Keep in mind that if you replace the pistons you will almost certainly have to rebalance the crankshaft. Just need to keep that in your budget.

 

I think most of those options on sites like Summit and Jegs are automatically generated and really have little to do with the item you are looking at.

I have said this a bunch of times and will say it again . . . . NEVER go by the technical information on a retailers web site. There are more mistakes than facts there. Go to the actual manufacturers web site. Its a little harder to search but you will get the correct information.

 

Cutlassefi has the best price on those pistons. The Speedpro are an old design made in India. They need double the piston to wall clearance as the Mahle pistons. You will also be over 10 to 1, needing a big enough on the cam to prevent detonation. Yes, it will need rebalancing, these pistons are much lighter than stock or the Speedpro forged.

 

Actually they run exactly the same clearance .0025" minimum

The Mahle pistons are obviously a much newer technology piston but to be honest with you there is nothing wrong with the Sealed Power ones for most uses. Where they are made means absolutely nothing in this day and age. Sadly there is plenty of "junk" that is made right here in the good old USA

 

Many guys on multiple sites have shown their scuffed pistons at the .002" the instructions state. The box actually says. 001".

 

Which box ? The Mahle catalog and the Sealed Power catalog both show .0025" minimum clearance for their respective pistons.

 

Ok, after a bit more measuring, it's becoming clear there is no way of re-using the original ones. Bores are already measuring 4.060 and by the time you hone your getting near 4.062 and with the original pistons at 4.055 ish, we're talking piston slap city.. I'll still have the machine shop verify for sure. I'm actually looking for a shop now..long story
Sounds like the Speed pro L2320's are going to yield too high a compression ratio so I also found speed pro L2321f that has a small dish but from my research they'll come out to amost the same price as the what i can find the Mahle's for. Guess it will be time to contact Cutlassefi. I also just realized he's about 3 hours away from me!

I guess I just didn't realize the 'oldsmobile price factor'. Last three engines I've done were a '67 camaro 327, '98 4g63 turbo, and the 318 in my '69 baracuda conv.

Time too buckle down!

 

Then they changed them. It wasn't long ago, someone posted their Speedpro forged piston box stated .001" minimum clearance, for an Olds 350, I believe. The instructions in the box supposedly say .002". There are many guys with hot running motors, running what the instructions said. Are you talking the forged pistons?

 

The Mahle pistons will be CHEAPER than the F2321 through Summit once you buy rings. It was $650, I believe for pistons, pins, locks and rings for the Mahle pistons from Mark. Make sure to ask your shop a lot of questions. I didn't ask my shop if they did in house balancing. They also said one month or 2. It took nearly 7 months for them to do the block. It is nearly 9 months and probably 10 before the crank is back from another shop where it is being internally balanced.

 

I am talking about the two part numbers actually discussed here. The L2320F Speed Pros and the 197815300 Mahle. They both say that .0025" is the minimum clearance. There are some forged Speed Pro Chevy pistons that can be run at .0015 without any problem. I wonder if that is what they were looking at ?

To be honest with you its been a long time since I have seen a piston that was not machined from the factory for the correct clearance. I personally prefer to run at least an extra .0005" on everything. That gives a little fudge factor when guys dont get their tune correct.

 

L2320 and L2321’s need .004 minimum. Otherwise you stand a good chance of sticking them.
Plus either Speed Pros are really heavy, and come with antiquated ring packs. You get what you pay for.

 

There is actually a range, for racing, Mahle suggests .0010" more clearance or .0035" maximum clearance. Mine will see the track, only occasionally and should run plenty cool, so it was bored to 4.100", which should give .0025". There have been issues with motors running hot with Speedpro pistons at .002", which is supposedly the instructions in the box. I saw the pic on here of the box showing. 001". One guy just assembled a 403 with the now discontinued .030" Speedpro forged, he went with .003". He hasn't even started the motor yet, time will tell. Is it worth the risk on thousands spent?

 

I Went with speed pros on my current build because this engine wasn't worth really nice pistons lol. For the 350 dollars or so I spent it was worth it. I went with .005 piston to wall clearance. This engine will be spun to 7k. I ran the same pistons on the street on my old street strip combo with no issues. Perhaps a little heavier and outdated ring pack but the pistons have gotten cheap to the point they are more viable for a budget build over something else more contemporary in design.

 

Nothing wrong Copper when you trade parts, add good used and racing is your main priority. I wanted 91 octane compatible with a decent timing curve, fairly smooth idle, excellent ring seal, oil pressure, decent fuel economy, low rpm cruising and be able to drive across Canada if need be. The LF2320 are $200! cheaper than the LF2321 on Summit.

 

With the flat tops and then sitting .025 in the hole you can probably get 9.6 to 1 ish range which will be fine for 91 depending on over all combo. I have ran 91 in a pinch with no issues with 10.25 to 1. On the current engine I had to buy new flat tops .

 

It is possible without a doubt on 91. By the time his heads get a clean up cut, the deck milled, my block needed .016" removed to straighten it as an example, it easily puts it at 10 to 1 with the LF2320 with the 4.091" bore needed. I had to run very cold plugs and retard the timing with 9.6 to 1 with .067" quench and a 204/214 cam with 91. It wouldn't try it with any less than a 220 duration cam in the same scenario. The Mahle pistons will give me 9.5 to 1 with 68cc heads, .039" quench and a 218/218 roller cam. It all depends on what he is comfortable with and his budget.

 

The voodoo cam I ran in my last engine has to be one of the nicest idling cam for the size and that can still produce vacuum. It was a 527/539 227/234 duration I wanna say. Yeah with that little of a cam engine won't be happy.

 

Copper, what vacuum did that cam produce? Also what rpm was the idle set at? Another thing to remember is the high octane lead fuel is gone that a 70 high compression ran with the under 200 duration factory cam. A careful tune and 93 probably works Ok on that motor being around 9.6 to 1 with better quench, not the 10 to 1 advertised. Also the factory converter stalls at 1600 rpm and his car will probably have 2.56 or 2.73 gears. Even with a mild cam, if he keeps the stock gearing, a 2000+ stall converter will be needed to get out of it's own way.

 

Guys, maybe this will make it easier. I'm not looking to build a street racer. I have 4 other classics and one is already a little built up. The story with this one is my dad bought this car in '72 (hence my user name). I drove it to high school as a senior in '81 and a little bit after that. In 2017 my dad passed it on to me. It's a '71 cutlass supreme conv, 4bbl, viking blue, black top and interior. The only work it ever had done was some sort of valve job my dad had done in '78 and an ok paint job done in 2007. I replaced the carpet and front seat upholstery in 2018.
I'm keeping the car as original as possible but just want to up the power a little bit. Leaving the rear gears alone (still have to check but most likley 2.73) No transmission work. Keeping stock manifolds (intake/exhaust). Only going to dual exhast and the cam profile i'm looking at is mid 400's lift, duration @0.50 around low 203ish intake, 212ish ex. example: crower 56260, duration 203/212 lift 448/450 112 lobe separation.
So now you see why I want to stay away from 10:1 compression. I just wanted to come up from the original 7.9 or 8:1 compression it has stock. 9 or 9.5:1 should be fine. It looks like the Mahle's should do the trick.
I would even leave the stock pistons if I could but I just can't swallow that low compression.

 

You have to decide what you want to do. Don't intermix advertised compression ratio's with actual compression ratio's. You may also end up using a thicker head gasket that will drop your compression ratio. MEASURE how deep in the cylinder that the top of the piston is. Re-surfacing the block and heads will probably not change the compression ratio much with the thicker head gaskets available now.
I would be more concerned with the loose fitting cast pistons. Don't be afraid to ask Cutlassefi for his thoughts. You may even decide to have him do some of the work, he is "just up the road" from you.

 

With that cam, definitely go with the Mahle pistons and good quench. The only other option which more money unless you can find a dealer, is the 12cc Dish DSS Racing piston, the coating is extra and has 1/16"/1/16"/3/16" ring packs vs the 1mm/1mm/2mm Mahle. More oversizes are available as well as wrist pin and ring packs options. https://shop.dssracing.com/product_p/K1-6020-4057.htm I would highly suggest an affordable converter like the Jegs 2000 to 2300 stall.https://www.jegs.com/i/JEGS/555/60402/10002/-1 My 70S with 9 to 1, a 214/214 cam and 2300 flash stall converter with 2004R would easily do 1/4 block burn outs with the 2.78 gears. My Daughter ran it low and fried it, went out of town without telling me. I put the stock TH350 in with the stock 1600 flash stall converter. It could only break the tires free on gravel, took all the fun away. Good luck.

 

Thanks for all the input. Now I need to find a new machine shop.

 

Is the first gear ratio the same in the 200R4 as the TH350 ?

 

No.

TH350 = 2.52:1
TH200-4R = 2.74:1

 

Yes, slightly better. These transmissions have the best gear ratios out of nearly any 4 spd OD automatic I have seen. Also the common D9 2004R converter stalls 300 rpm higher than the basic TH350 converter, around 1900 rpm. The higher stall is the bigger difference, with a higher peak torque rating of an upgraded cam.

 

So, the 200R4 has about a 10% lower first gear than the TH350.

 

Ah yes, the common mistake most people make when selecting a cam. Bigger is not always better. The more duration, the more low end you lose.

 

True but the stock converter is tolerable with a stock cam, anything above, not so much.