72convert455

I have a '72 Cutlass Supreme convertible that has been in the family since 1973. My dad used it as his daily driver until his death in 2010. The Rocket 350 has always been maintained and to my knowledge never rebuilt.

After my dad passed the car sat six years in my mom's garage and it wasn't until a couple of years ago I pulled it out and started driving it again. I've had some minimal work done to the car, such as new brakes and exhaust, but nothing has been done to the motor.

Last year the engine started behaving badly in that it stopped running. I took it to my local mechanic and after inspection was told five out of the eight pistons had low or no compression and the block might be cracked. My thought is to pull the motor, strip it down, and send the block to an engine shop to see if the block is for sure cracked.

If it is then I'd buy a new block and rebuild the engine so the car stays original. My wife thinks I'm nuts and I should forgo the rebuild and put a modern engine and EFI into the car.

My thinking is to stick to the small block 350 because the car will mostly be driven on the weekends and not be a daily driver.

My dilemma is I'm not sure which engine to source for my car so any recommendations would be appreciated. Any suggestions other than the small block 350 would be welcomed too.

cutlassefi

Fun71

I would suspect a valve issue before a cracked block. Unless the engine was out in freezing temperatures or was severely abused the block shouldn't crack easily.

redoldsman

If it were mine, I would go with your original plan. If the block is cracked I would then look at putting a 403 in it. It is a bolt in and you would be able to get a little more power from it.

72convert455

No freezing temperatures in SoCal and I'd like to keep my budget to around $4k. Would there be any issue with motor mounts if I use a GM or Chevy motor?

redoldsman

You can put a Chevy motor in by using Chevelle mounts since the frames are the same. You will not be able to use the current transmission since the bolt pattern is different. You will also have to change wiring since the starter is on the opposite side. You will be money ahead by rebuilding the Olds engine and your car will retain a better value. No way I would do that.

72convert455

Thanks, redoldsman for the sage advice. The rebuild option sounds the most fun to me.

Sugar Bear

With the family history, originality and age of the car I would focus on fixing the original engine, just my opinion. It's your car to do as you wish but it is only original once.

If the engine has not been frozen, severely overheated and shows no specific symptom of a cracked block, the chance of it being cracked is VERY, VERY low...almost nil.

Engines don't run ok one day and suddenly need a rebuild with five low compression cylinders the next day. I'm making a long distance guess that the timing chain jumped. Have someone that works on old cars do a leak down test to pinpoint the compression issue, it may need a timing chain, valve job and major tune up. If it needs a rebuild use someone that has OLDS experience.

Good luck and please keep it posted!

olds 307 and 403

Cutlassefi, Mark Remmel brought a modern piston from Mahle that would be perfect for your build. Mark even brought a 4" stroker crank to the market for the Olds 350. Follow his advice for goals, he will advise you the best places to spend your money. The Olds 350 is a very good motor, chances are it isn't cracked.

redoldsman

If your block is not cracked, you should be able to build a nice 350 for $4,000 depending on how much labor you have with your mechanic.

OLDSter Ralph

I vote to keep it all OLDSMOBILE. Future value and family ties are big considerations. The 350 can be "warmed up".

droptopron

Re the Chevy option, the engine will be cheaper to build (or buy a crate engine) but everything else will cost more. You'll need different frame perches, motor mounts, exhaust manifolds or headers, etc. For all your trouble, the car will be worth less. You could make an argument for an LS, but that would be beyond your stated budget.

Another option would be to find a running, complete 455 Olds and drop that in. Its an easy swap. Just need different manifolds or headers.

Re Fuel Injection, with the modern EFI throttle bodies available, that can be added to just about any engine including your 350

Sugar Bear

FWIW, you may want to confirm that the 350 in it is original.

oldcutlass

I agree with Sugar Bear, its probably the timing chain if it never was changed. This can be confirmed by turning the engine back and forth and measure how much play is in the distributor shaft.

72convert455

Thank you, everyone, for your suggestions and advice. I'll check the timing chain to see if that's the cause of the problems. The motor is the original to the car.

72convert455

A little update on my Rocket 350. I decided to keep the original engine with the car instead of replacing and possibly encountering more headaches. I dropped the car off to my mechanic and told him what I thought the problem was. He performed a compression check and revealed cylinders 1, 2, 3, 6, 8 have 120psi; #4 has 115psi; #5 has 125psi; and # 7 has 0psi. Total engine rebuild is needed. His estimate was $8k, which is high because it takes into account any unforeseen problems. Probably walk out the door paying $6k. The car is back in my garage while I figure out the next steps.

Fun71

While I don't disagree with rebuilding a worn engine, I do question a "total rebuild" recommendation because one cylinder is dead. It could be due to something as simple as a burned / stuck valve or a blown head gasket.

Greg Rogers

My guess is a burnt exhaust valve in #7. If you want a "new" engine than go for a complete rebuild. If you want it to be as it was why not just find out for sure what the problem is with #7? How many miles on it? 50,000 or 250,000? That makes a difference..

OLDSter Ralph

Get a mechanic that knows Oldsmobiles............. and avoid the Mondello in Pasa Robles, California.
Good luck

brownbomber77

There's a badass 403 down here in south Florida. Mark built it and it's a drop in motor.

AZ520

If you are trying to save you should consider a mabbco long block and swap the engine out yourself.

Tedd Thompson

It's got to be something more than one bad cylinder if it quit running all together as stated. Though poorly a engine will run with those compression numbers on seven cylinders. If a valve is totally burnt it may backfire through the carb but in my experience it will still run. Some how we aren't getting all the facts or all the condition info...... Tedd

olds 307 and 403

As said, it is usually a valve issue with 0 psi. Still it probably needs a total overhaul, other numbers are fair but not great. Make sure he uses better pistons than the 24cc dish cast replacement pistons, unless you want 7.5 to 1 compression. Either Cutlassefi's Mahle 10cc, ideally or the heavy, ancient Speedpro 6cc forged LF2321 pistons. Also upgrade to a mild cam which Cutlassefi can also provide. Quality parts go a long way.

droptopron

JMO but for a basic engine (no disrespect) I think #'s matching is overrated. Nice to have but not a deal breaker. Consider putting that engine to the side and finding a good running engine to drop in. A nice 455 would be sweet.

olds 307 and 403

Another option, finding a good Olds 350 is easier here, the 455's are over priced, knocking or both in my area.

72convert455

Greg Rogers - the odometer reads 90,351. I was mistaken in my first post when I told you the engine doesn't run, it does but rough.