Compression tested the 352 in my 1967 F100. Averaged between 90 and 125psi with most in the 115psi range and two cylinders at 90 psi . So looks like a rebuild is in order. I'd like to upgrade it while its out, is it worth messing with the stock heads?

 

Was throttle blocked open when testing? Did you try putting a squirt of oil in the cylinders and re testing? Did you test a warm engine or cool?

Stock heads are fine. Unless you have money to burn, in which case you can give some to me. I have a set of heads I need to have rebuilt....

 

Compression testing can give you an indication of the engine's overall 'health' but, it's not as comprehensive or as accurate as a leak down test.

No engine will be 100% leak-free but, a race engine shouldn't leak more than 10%. A street vehicle engine shouldn't leak more than 20%. Beyond 20%, an engine has a good bit of wear that will need to be addressed soon.

If the leak-down tester has two gauges, the gauge closest to the incoming compressed air side will indicate the amount of supplied pressure coming in. The gauge closest to the engine will indicate the amount of pressure loss through the engine, relative to the pressure reading on the supply air gauge.

If the static pressure of the supply side gauge is set at 100 psi and the engine side gauge is reading 85 psi, then you know that cylinder has 15% leakage.

During a leak down test, if you hear air coming out of the carburetor or throttle body, you know that the intake valve is leaking on that particular cylinder.

If you hear air coming out of the exhaust pipe, the exhaust valve is the culprit.

If air is coming out of the PCV, oil filler neck or the dipstick tube, then you know the leakage is past the piston rings.

If air is bubbling up through the coolant in the radiator, then you know there's a problem with the head gasket.

A compression gauge will not tell you these things and you won't have the time to listen for them with a compression gauge check.

This is a leak down tester I made back in the '90s.




A couple of links on how to use a leak down tester.

How to do Cylinder Leak Down Testing- Car Craft Magazine

https://mobiloil.com/en/article/car-...-leakdown-test

 

If it was just one cylinder that was down on a engine that had less mileage then, I'd do a leak down test.

But most guys don't have a air compressor just to do the leak down test.
And it's all money well spend, but if cash it's in short supply like for so many of us.

Then second best is the old standard way of testing that gives you a good indication which cylinder has failed..

When doing a compression test on all the cylinders the same way and you have a few cylinders that come up way short like yours did don't really matter if the carb was blocked open or not since all cylinder were check by the same method.. IMOP

Orich

 

While Ultra is correct a leak down will tell you more, 2X Orich. I went through 2 1/2 years of vocational Auto Mechanics class in High school. Both class and our own shop time. I graduated in 1975. Then I was taught if the difference between cylinders in a compression test is more than 10% the engine needs help. After all, it is a balance assembly. While in the real world I may stretch that to 15%, 90 to 125 is really bad. Doing a wet and dry compression test will generally tell if the problem is in the heads or rings. A bad head gasket is in the middle but that is obvious elsewhere. I don't mean to be disrespectful to the leak down fans but at 90/125,if it's in the heads, I wouldn't care if it's intake or exhaust valve. All of them would get ground.

 

No disrespect taken. I have a compression gauge too but, I just prefer the leak down tester. It gives me a greater opportunity to study each cylinder, to pinpoint exactly what offending component may be causing the excessive leakage problem.

It also gives me an exact percentage rating of leakage in each of the cylinders. My compression gauge can only tell me if the pressure in a given cylinder is high, low or somewhere in the middle but nothing else.

 

Thanks for the input guys. Definitely a help to have so much knowledge in one place. It being the original engine and it having never been out of the truck (its very "original") I'm sure its over due for some attention.

 

Do check also check in your repair of the valves as your pulling the engine down is a careful inspection of all the moving & nonmoving parts so you don't miss anything.

But if you don't know what your looking for then it's kind of hard to know what's good what is not or needs replacing.

1. Carefully remove the valve train and label each one. Then slide each of the rocker arms over and inspect the under side of the rocker arm shaft for wear replace as needed.

2.Remove the push rods, these may be reused so keep track of these as they will need to go back in the same location facing up or down the same way removed.

I use old wooden cloths pins clipped onto the upper end for ID'ing which end is up.
Once you have them cleaned them roll each one checking for any bent one's if one is more then .020 then replace them all.
But these come in different sizes so take this up the the shop guy doing your head work.

3. Best to replace the timing chain set now is a smart idea now don't wait til it falls apart and jumps timing. Buy a double roller set not a oem one.

Have the shop shave the exh.port bosses so you'll have a good manifold sealing face.
If using the old exh logs it's best to have them shaves also to true them up also.

Now don't use any exh. manifold gaskets, but use the exh.heat shields this was what ford used with lock tabs.
The oem lock tabs are a pain to remove when not pulling the heads.

So use some type of locking grade 8 bolts, no lock washer the heat will mess them up and the bolts will work loose.
Or every 6 mos. check bolt TQ 12-18 lbs
Use some anti seize lube on all the exh. bolts before installing the manifold.

Well got a be something I'm forgetting someone will chime in and let me know.
I'm sure of that..

Orich
Here mine

 

I have read the OP's compression numbers and I don't see anything alarming. It is a tired worn old motor with 90-130 psi of compression but if it is just a daily lugger and is cared for it will probably give years of useful service before a rebuild is required.

 

2x Cougar John. I think the EPA got rid of these motors because they could still limp in a load on three working cylinders.

 

No one mentioned anything about a vacuum test yet...

 

Ah. True story. A low reading on the gauge would show there's a vacuum leak. Like say a ruptured diaphragm in a brake booster. A bouncing needle on the vacuum gauge would indicate a intake valve that's not sealing for various reasons.

 

A slow paced 1-2 variation indicates an ignition issue.

 

Well I want to freshen the engine up anyway so this just gives me the excuse to do it sooner I picked up this truck for $1300 and I have set aside some funds to do the engine knowing it would likely need a rebuild before a 4bbl and some headers :-)

 

Nice truck when you say freshen up are you going to do a full on rebuild?