leakdown cylinder reading experience needed
 

did a leakdown test on a 351 crate motor, using 60#'s, and all were reading in the 54-56 range, except one. it read 50#'s.

my question, would a difference of 4-5#'s indicate a blown headgasket, cyl #6 by the way? i've experienced this before but the difference was quite a bit more.

i have a seep on drivers side between block and head, what i thought was at the rear, but could be #6 and running back on the crack.

cooling system built enough pressure to blow hose off radiator, although hose could have been loose, and there is no h2o in the oil upon draining.

so what do you think? tear the head off, or go with it?

i had attempted to run bars leak but the regurgitation was quicker then the seal.

none of the plugs looked worse for the wear, and you couldn't pick out #6 if you had to.

Leakdowns are expressed in percentage points. Since you used 60# to test you should multiply all results by 1.666666 to get % numbers.

eg 60 x 1.66666=Approx 100lb/100%
Therefore your 55# = 91% & 50# =83%

Head gasket coolant leaks wont show high % numbers , but any individual cyl that cause cooling system pressure requires further investigation.

When checking cyls try to bring piston up in direction of rotation to TDC, you can get variation in readings if piston is rocked at TDC. Also doesnt hurt to re-test at BDC with valves closed, only way you will find cracked cyl walls etc.

Should have added that you therefore have 9% leakdown on the 'good' cyls & 17% on that bad one.

hadn't thought of checking the cylinders with the pistons at the bottoms, but......

i went out today and was getting ready to take at least the one head off, debating the two, and decided i would do a retorque.

a guy in the know preaches loosening the bolt in turn on the last step and torquing the full value in one sweep on rebuilds, so i thought i would try this and see if i could bring the value up. i wasn't expecting great things but.......

the leakdown after the retorque showed normal values, and i had checked it again before to make sure the cylinder was low.

talk about tickled pink! saved me a ton of work! i'm going to retorque the rest and call it good and be on the road again sooner than i thought.

so you guys blowing gaskets and coolant can try the above and possibly save yourselves some work. will see how the longterm works out, but willing to take the risk for now.

good day.

There is a valve open at both Bottom dead centres.
Exhaust is open on one BDC, and intake is open on the other BDC.

I was assuming that the rockers would be removed while checking at BDC , a dangerous thing to do around CC members- assume nothing:):) I wish Vector luck with his retorque, once coolant has started to 'track' across the Headgasket ( esp if its of composition type ) its usually on a one way trip to failure.

i know about the valves open at bdc.

jac mac, i thought about the leak once started, hard to stop, but i'm not risking anything except the possibility of saving a lot of time. there was one bolt on the bottom of the cylinder that broke loose easily, so am thinking that was the one. have to repair the radiator now.

also, it is a low compression crate motor.

You can check for that coolant leak by while doing the leakdown check by simply removing the rad cap and watching and listening for air bubbles coming out.
If you have blown the rad hoses I think you are leaking ,easy check,but maybe you got lucky with the retorque .

Not the correct air pressure for leak down
 

vector1 100 psi with 100 and 120 psi gauges are the correct way to check for compression. I have done FE heads checking for porosity with 75 psi and had no problem. Check the pressure to 100 and out comes the leak. This was in a combustion chamber. The first time the motor would have run, the head would have cracked open like a water melon. If you are retoqueing head bolts or studs, you shouldn't back off them to 0 and then crank them down to spec. Your leak may have stopped because you are crushing the gasket tighter after a couple of heat cycles. Some gaskets are OK with your idea and some will just blow out. Did you oil the bolts or studs before the retorque?This will effect the final number also. I hope that just the bolts or nuts where low on the head torque spec. When you are doing the retorque follow the procedure for starting in the middle and working out on both sides. If you start at 1 end and work across the head you can pinch the gasket on the started side and not have a good seal on the other end. Your problem may only have happened when the motor was cooling off. Are the heads Aluminum and the block iron?? There is an expansion issue with these 2 metals of about .0012-.0015". Good luck and watch the oil for cream and the breathers in the valve cover. Make sure the valve cover has NO pudding in it. If it does, you have washed the bearings. Change the oil and filter a couple of times and watch the oil pressure on a cruise. Rick L.

how's this look for a radiator?