Compression - Is 100 psi too low for a 4lo motor ?

[fredswat]

I have been rebuilding my 350 which has been standing for a while. Put a rebuilt crank in and new seals. Replaced the head and fitted a steel gasket. The barrels and pistons appear fine. Torqued up the heads got it ready to try to start it(fitted carbs, cables etc). I noticed it appears to be weeping water from the headgasket . I decided to compression test it before trying to start it. Both cylinders are the same 100psi. Is this too low? I am going to drain the coolant and apply a little bit of silicon to the headgasket to see if that stops the water leak. I will test the compression again when I have done this. But thought I would get some wisdom from the experts

[stusco]

rdlccrazy.proboards.com/post/325446/thread

[jessy03]

I’d check the head to make sure it’s flat before I smear it with sealer, if you feel the need to use a sealer use a spray type- Permatex or holymar but if everything is flat and clean you shouldn’t need any,100 psi is fine if it starts and runs ok I’d leave it, on a rebuilt motor you would probably see 115-120psi if it’s built correctly.
Jess

[mcludd]

My thoughts on compression testing is that the gauges supplied with the kits are not exactly the best quality (unless you do have a quality one thats calibrated regularly) so the actual figure cant really be entirely trusted, I have 2 sets which both read different on the same cylinder however what I do find useful is to use them to compare cylinders ie if you have 110psi on one and only 85 on the other that is a pretty good indication of a problem, generally if both cylinders are within 5 psi of each other Im happy

[fredswat]

Oct 21, 2020 18:39:55 GMT 1 jessy03 said:

I’d check the head to make sure it’s flat before I smear it with sealer, if you feel the need to use a sealer use a spray type- Permatex or holymar but if everything is flat and clean you shouldn’t need any,100 psi is fine if it starts and runs ok I’d leave it, on a rebuilt motor you would probably see 115-120psi if it’s built correctly.
Jess

Thanks

Its not had a re-bore or rings as they looked ok.  The head is a new skimmed that has had the combustion chambers re-machined to std volume by an engineering company so it should be flat. I will take the head off and have a good look at the barrel tops and see if I can see where the water is coming from.  I have ordered some high temp silicon sealer (Permatex) which I will probably use conservatively when I put the head back on.  Any thoughts on steel gaskets versus conventional ones?  

[67kev]

For what it's worth, I've only ever had leaks with the multi-layered metal head gaskets ..... the conventional type have never caused me any probs.

[bare]

It's the 3 layer types that have proven Best in my works. That's a surprising claim TBH.

[JonW]

Oct 22, 2020 17:50:43 GMT 1 bare said:

It's the 3 layer types that have proven Best in my works. That's a surprising claim TBH.

Same here, always got for those, Yam went to the composite version and then went back to these. Must have been a reason.

[fredswat]

Oct 23, 2020 7:41:06 GMT 1 JonW said:

Oct 22, 2020 17:50:43 GMT 1 bare said:

It's the 3 layer types that have proven Best in my works. That's a surprising claim TBH.

Same here, always got for those, Yam went to the composite version and then went back to these. Must have been a reason.

I have switched back to a conventional one for the moment as it was the steel one that leaked albeit without silicon.  I will see how that goes.  I am waiting 24 hours before putting any coolant in to make sure the silicon has gone off as per instructions. Here's hoping

[bare]

3 layer types can be reused.. true. For as long as the stamped ridges in the two outer layers are still ridges.
Spritzing the used gasket with Rattle can LACQUER does a good job of resealing.

[JonW]

Can also be sprayed with copper spray.