New pistons New Rings

[Daveyyc]

Replaced the pistons, rings and rod bearings, inspected the valves which appeared to be reasonable. How long should I run the engine on the test stand to run in the rings and expect a reasonable increase in the compression. Currently I have had the engine running 45 minutes and the compression is consistently 55 psi.. Which I consider pretty poor.

[KiWinUS]

Zero Tim at all. Runstand is the worst thing you can do to run in an engine. Engine needs to be under load The best way is to install and run it hard and under load in your vehicle.
55+ years building flatheads and race engines at the highest levels in racing. You can possibly glaze bores running on stand then it may never bed in. Just trying to help. No skin off my nose if you choose not to take my. Cheers

[Bored&Stroked]

As Tony said . . . an engine stand is just to warm the thing up, check oil-pressure, check for leaks and then shut it down. There is no load on the piston rings, and they will not seat. Hopefully you have not glazed the cylinder walls - my guess is not. You want to put it in the car and run it full-bore from start to higher RPMs - do this a half-dozen times and hopefully you'll see the compression come up.

Also, "inspecting the valves" doesn't tell us much . . . unless you tested them under pressure/vacuum, you really have no way to know they are sealing well. Did you lap them or do anything to the valves and seats? Did you check your valve springs on a tester to see what pressure they had on the seat?

Pistons/Rings: Did you just put new ones in the old bores and not bore/hone the bores for an oversize diameter? If you put new stuff in old bores, it is highly likely the bores are worn and probably also have taper . . . which makes it hard for the new rings to seat. What was actually done in this area?

[Daveyyc]

The cylinders were honed. The original pistons and rings were .030 OS and appeared to be installed some time ago but with very little wear evident. I replaced the pistons and rings . 030 OS when I found a couple of areas around the wrist pin retainers broken away. There was no scoring in the cylinder walls. I did not make any measurements of the valves ( hindsight is always 20/20). Opening the “rebuilt” engine in the first place was to trouble shoot cylinder 3 that had no compression. This turned out to be a valve stem keeper incorrectly installed

[Bored&Stroked]

Well, it sounds like the pistons/rings should do the trick - as long as the cylinders are "round", then the rings should seat under load.

What we can't know is the condition of the valve faces and seats. Once you put some load on this and the rings seat in (hopefully), then you'll know if the valves are good or not. If the engine was rebuilt and didn't have any time on it, maybe the valvetrain is good . . . you'll know shortly!

Best of luck!
B&S

[flatjack9]

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I think you have something wrong if you only have 55 psi compression. Do a cylinder leak down test to determine where the problem is. Try oiling the cylinder walls and retest to see if the compression comes up.

[Ggmac]

Check your gauge . Make sure it’ll register accurately.

[Bored&Stroked]

Originally Posted by flatjack9

I think you have something wrong if you only have 55 psi compression. Do a cylinder leak down test to determine where the problem is. Try oiling the cylinder walls and retest to see if the compression comes up.

Yep - a leak-down test is the best way to isolate where your compression issues are coming from.

[Karl Wolf]

Do you have the throttle opened all the way when you did you test??
If there's not been a very recent valve job, I would do one.
In my opinion, this is the most likely thing to wear first in a well treated engine...

Karl

[ThirstyThirty]

all 8 cyls are only 55 psi?

[Gene1949]

Long enough to set the timing and the carburetor to idle
Varying RPM's for 1/2 an hour on the street, Load and unload the engine monitoring the oil pressure and temperature.
Then wide open throttle first and second, then drive it like you stole it from there on.

[Tim Ayers]

I'll add that I think it's a good idea to change the oil after running it on the street hard while breaking it in. Even in the cleanest of motors, you'll be surprised how much break in gunk gets washed up in the oil after the first run.

[Gene1949]

Originally Posted by Tim Ayers

I'll add that I think it's a good idea to change the oil after running it on the street hard while breaking it in. Even in the cleanest of motors, you'll be surprised how much break in gunk gets washed up in the oil after the first run.

Great minds think alike, I just forgot.

[hueyhoolihan]

if i installed new rings, pistons and rod bearings and found i was getting 50PSI compression i would immediately suspect that i should have had a valve job done. of course, i would run a few tests to verify my suspicions before taking any action.

[Daveyyc]

Thanks for all the input. The fact tha All pistons registered the same pressure is very odd.
Next step to check valve to lifter clearance before taking the engine apart again.

[Flathead Fever]

If you checked the piston gap with a micrometer and a feeler gauge on the rings and it was all good, then I would consider something else like maybe the timing gear is off. Very strange that all of the cylinders would only show 50l bs of pressure unless something was off affecting all of the cylinders. I can see having a burn valve or two but not every single cylinder. Maybe the cam gear is out of sync. Just a guess.

[tubman]

There is a member on the H.A.M.B. that had a problem similar to this several years back. If I remember, it was the result of a mismarked timing gear or some such. Perhaps a perusal of his threads from that time might be in order.

[ThirstyThirty]

Originally Posted by tubman

There is a member on the H.A.M.B. that had a problem similar to this several years back. If I remember, it was the result of a mismarked timing gear or some such. Perhaps a perusal of his threads from that time might be in order.

I have seen it before! customer got a bunch of engine parts from me... then took it to someone he said was a reputable repairman. V8. customer wanted to go run brackets on Saturday night. first few runs were slower than the tired, stock motor!! I kept telling him!!! I kept telling him... "sounds to me like incorrect assembly!!" "should run ideal!" but finally he took to another shop... and had to pay more $$, young guy, liked cars/trucks... young family... and the new shop tech found the problem!

timing gears one tooth off!

[Daveyyc]

Would it be correct? If the timing gears are one tooth off, the lifter to valve stem clearance would also not be correct!

[GB SISSON]

On the whole you can consider it's good that it is not installed in the car yet. Not an awful job to pull the heads and intake and get at the cause of the low compression once the tests suggested here are performed.