Fletchmo72

So I'm trying to get the most out of my pitiful ultra-low compression '72 350. The P.O. had it rebuilt with the dished pistons (I cannot fathom why anyone would do this). With only about 30k miles on the rebuild, my compression runs from about 100 to 141, with most other cylinders in the 120 area. Must have been a crappy job.
I thought it might be worth the effort to have the 7a heads redone, and replace the head gaskets with the thinnest ones available to boost compression.
My question: How should I tell the machine shop to do the heads? What size valves, etc.?
BTW the car runs excellent. Just doesn't have the power it should. Some day I might replace the pistons, but would probably just go with a 455 instead.
Just wanted to add that $900 was the bill for machine shop on the rebuild--this was in 1996 dollars. I assume that's enough to have done a valve job and honed the cylinders.

Olds64

That's quite a bit of variation in compression numbers. Did you squirt a bit of oil in the cylinder to get those numbers?

Fun71

I'll let others with more knowledge comment on the head work.
I just thought I'd mention that you can get a camshaft with earlier closing events that will build more cylinder pressure.

olds 307 and 403

As said, 40 psi is a lot. Make sure it isn't rings,the oil trick should help eliminate where the issue is. Honestly, Cutlassefi's piston with a 10cc dish will give you proper compression. See if Smitty from M+J Performance can get .011" head gaskets. W31 valves with the bowls opened is the norm but kind of a waste on a 7 to 1 350. I would talk to Cutlassefi about the Mahle pistons or one of his really tight LSA custom cams for low compression engines.

Fletchmo72

I have one cylinder that is significantly lower than the others. It build up to 145 psi with oil, so I know it's the rings on that one. I haven't checked the others with oil yet.
On the receipt from the P.O.'s engine rebuild it shows "HP cam kit," but gives no other details. However, at 1,000 rpm the engine pulls 18-1/2 vacuum, so I'm thinking the cam can't be very big.

Olds64

What is the reading on that cylinder without oil?

olds 307 and 403

Probably the generic 204/214 cam, mine had similar vacuum. Get the 10cc Mahle pistons, add 2"/1.625" W31 size valves with bowls opened, Performer Plus springs, if it is the 204/214 cam, make sure the head and deck are straight, maybe an align hone as well. Figure out what oversize pistons you have, assuming .030" oversize but who knows. Either way the cylinders will need a bore and hone or maybe just a hone depending on your current pistons, the Mahle pistons have different oversize's than your current garbage pistons.

Fletchmo72

103 psi

Fletchmo72

What's that gonna cost me??

deadeyejedi

youll be in it for 4 grand minimum! now may be the time for the 455 ,but that will come at a cost as well. machine shop cost in my neck of the woods are up there.if it runs good you might want to just enjoy driving it for a while until you can figure out a plan .

olds 307 and 403

It depends on many factors. If the block was decked properly, the heads rebuilt properly, new guides, proper valve job along with the rods, crank and bearings are good condition and reuseable, considerably less. You may get away with a cylinder hone to the proper size, press the old pistons off the rods, install the new ones with rings, balance and new gaskets. It all depends what you find inside. Best case scenario, I say around $1000 to $1500 and your motor will perform much better.

Fletchmo72

Hope you're right! I do think everything else is okay. No knocks, no clicking, no smoke, doesn't use oil, good oil pressure, runs good. Nevertheless, I don't want to be without my car for the summer. I'll probably have the heads done and see what that does. I'll be able to inspect my cylinders and check them for out of round, etc.
More more thing: they ran a 160 degree thermostat in this car after the rebuild, and for the next 25k miles or so until I bought it. I wonder what effect running such a cold temp had on that rebuilt motor. (I run a 185 now.)

shiftbyear

Sometimes sealing up the top end like a valve job tends to blow out the rings and causes blow-by and blue smoke out the exhaust. As deadeye said, enjoy now and have a back up plan in case things go south. Maybe start working on a new bullet with all the bells and whistles. Good luck.

Fletchmo72

Wow, that would suck. Thanks for the heads up (no pun intended).