59A low compression #4 cyl
 

Purchased a Project 33 PU several years ago complete with 59 engine. Have decided to get going on completing the truck. Started with checking the engine for issues. Compression check shows all cyl except # 4 to be 80 to 95 lbs. # 4 cyl is 34-40 lbs .All these cylinders are original parts except, #4 cyl intake and exhaust valves have been replaced. The low compression in #4 does not appear to be a valve problem although I am going to replace them with VanPelt valves.
The cylinder walls are what you would expect in this original engine and all 8 cylinders are consistent. I also have new head gaskets.

Would you share your thoughts on anything you might try before I have to tear down the engine. ..... Thanks

Re: 59A low compression #4 cyl
 

You could replace the head and do a leak down test to find problem.

Re: 59A low compression #4 cyl
 

Since you've come this far, why waste a set of head gaskets on it - when you really don't know the issue. If it was mine, I'd take it all the way down - being careful to label where all the parts go (pistons, bearings, etc) - and check it out in detail. Depending on what you find - maybe you get away with new rings a cylinder hone and a valve job. If you're trying to do it on the cheap, then you might get away with some basic/simple stuff - all depending on amount of wear and condition of everything.

Me - I'm never that lucky and I'm too particular anyway - I always end up with a full rebuild! LOL

Re: 59A low compression #4 cyl
 

In pic #2, is that a burn mark at 11:00 on the exhaust valve?...... Mark

Re: 59A low compression #4 cyl
 

Don’t shoot me, but now is the time to tear this motor down and have it hot tanked and magnifluxed. You may have a crack in the valve chamber that you can’t see with the naked eye.most small cracks can be fixed. Good luck.

Re: 59A low compression #4 cyl
 

Thanks for your help and advice which is all good. I agree a full rebuild is what it will need eventually. My problem is I currently have one in the shop being rebuilt for my 35 PU and I'd like to use this engine shot term. FlatFord8, I'm quite certain that the mark at 11 o'clock is not a burn mark.
Question - If I had a stuck piston ring on cylinder 4, could that cause low compression and has anyone ever been able to free it with out pulling the piston. ?

Re: 59A low compression #4 cyl
 

I can completely understand the desire to not end up with two flatheads in the machine shop at the same time. I think for the time being, you should clean up the surfaces and re-install the head with a new gasket. Then, as "marko39" suggests, do a leak down test to find out where the problem is. Depending on what you find, I would button it back up and fire it up. You didn't say how long it had been sitting, but things can get gummed up over time. If the engine doesn't make any funny noises and has good oil pressure, using it may just loosen up anything that may be sticking. Proceed step by step, and there is a pretty good chance that you will end up with a good runner. (Don't forget the MMO and maybe some SeaFoam.)

Re: 59A low compression #4 cyl
 

Mr. Tubman, thanks for your response and idea's. Your advice is what I'd hoping to try as a Hail Mary fix. The engine has been out of service for at least 10 years maybe longer.

Would you mind sharing on how to go about a "Leak Down test ". I'll try a Forum Search as well.

Re: 59A low compression #4 cyl
 

Google might be quicker. Spray some penetrant around that piston for a few days and maybe it’ll free the ring....... Mark

Re: 59A low compression #4 cyl
 

https://www.mobil.com/en/lubricants/...-leakdown-test I found this in my stash of info...... Mark

Re: 59A low compression #4 cyl
 

Can you take a pic without the gasket on ?

Re: 59A low compression #4 cyl
 

Question: Was the compression testing performed wet or dry (oil in the cylinder for wet)? A wet test should tell you if it is a ring issue.

Re: 59A low compression #4 cyl
 

Thanks for the help everyone. I have removed the head and going to put the penetrant in the cylinder for a couple of days. I'll give it a short turn now and then. Flatford 8 ,Thanks for info on the leak down test. I'll give that a try after the penetrant.

Re: 59A low compression #4 cyl
 

A sticky valve (taking too long to close) could cause your problem too.

Re: 59A low compression #4 cyl
 

Thanks Mr Newland. A sticky valve might explain why I get different readings each time I do a compression test Ex.. readings of 20 PSI, 35 and 40 PSI