1968 Ford 4000 Compression Pressures

[FishDuck]

Hello again,

Measured compression on 1968 Ford 4000 diesel 3-cylinder engine and had the following results:
300 psi on two, 280 psi on one cylinder doing a wet compression test.

The service manual states 420-510 PSI +/- 50 PSI.

The tractor was hard to start and there are multiple issues I'm working thru. One was a bad fuel pump putting fuel into the sump so I cannot contribute the hard to start to low compression only. Also, was unable to test the output of the fuel pump but verified the injectors were providing good even atomized fuel (visually not on a flow bench). Prior to finding these issues, once you got the tractor running, even with all the improper things going it would keep running.

Wondering if others have seen lower than spec compression numbers like I have and what the outcome was (rebuild or let it eat)?

Thank you for your help

 

[used red MN]

Hello FD welcome to YT! Unfortunately it looks like your compression is just too low. That spec seems optimal and on the high side. Dieseltech a well known poster on here who repairs diesel injection pumps and is very knowledgeable on diesel engines suggests at least 350-400psi and you are not even at that. If you want it to start and perform reasonably well, time for an engine rebuild.

 

[Dieseltech]

Diesels will run at 275/300 PSI but don't START well when that low. I just had the head off my early Ford 5000 diesel that had 300 PSI and would not start in a 60 degree building. After having all new valve seats installed it now has 380/400 PSI and starts MUCH better cold than before.

 

[FishDuck]

Pulled the head off and measured the 3 cylinder's nominal bore. All were nominally 4.425". Manual says bore should be 4.4".

The pistons were stamped STD and seemed like too much room between rings and cylinder wall. Seems to me like someone rebuilt the motor with a cylinder 0.020" overbore and put the 4.4" Standard size pistons back in. I imagine this would negatively affect compression pressures...

Valves and valve seats looked good and the head cleaned up nicely. Unless someone advises differently my plan is to purchase a set of 0.020" over sized pistons and put it back together. Any other thoughts or comments on the above information?

Thanks
FD

 

[Dieseltech]

Pulled the head off and measured the 3 cylinder's nominal bore. All were nominally 4.425". Manual says bore should be 4.4".

The pistons were stamped STD and seemed like too much room between rings and cylinder wall. Seems to me like someone rebuilt the motor with a cylinder 0.020" overbore and put the 4.4" Standard size pistons back in. I imagine this would negatively affect compression pressures...

Valves and valve seats looked good and the head cleaned up nicely. Unless someone advises differently my plan is to purchase a set of 0.020" over sized pistons and put it back together. Any other thoughts or comments on the above information?

Thanks
FD

The piston top RINGS area will measure SMALLER then the lower skirt area will. Accurate piston measurements can't be made until pistons are out and on the workbench.

 

[Destroked 450]

Bore will need to get checked in several places to be sure it’s a .020 over bore or just badly tapered wear

 

[Bern]

The piston top RINGS area will measure SMALLER then the lower skirt area will. Accurate piston measurements can't be made until pistons are out and on the workbench.

You are correct, however if the pistons are clearly stamped "STD", then they should measure around 4.395-ish inches at the appropriate measuring spot.

 

[Bern]

Bore will need to get checked in several places to be sure it’s a .020 over bore or just badly tapered wear

A person could scrape away the carbon at the very top and measure above the ring ridge, that should be an accurate measurement.

 

[Bern]

Pulled the head off and measured the 3 cylinder's nominal bore. All were nominally 4.425". Manual says bore should be 4.4".

The pistons were stamped STD and seemed like too much room between rings and cylinder wall. Seems to me like someone rebuilt the motor with a cylinder 0.020" overbore and put the 4.4" Standard size pistons back in. I imagine this would negatively affect compression pressures...

Valves and valve seats looked good and the head cleaned up nicely. Unless someone advises differently my plan is to purchase a set of 0.020" over sized pistons and put it back together. Any other thoughts or comments on the above information?

Thanks
FD

If your measurements are accurate, then I agree with your conclusions. Check your valve recession while you're there, should be in the .060-.080" area. If much greater than that, this will cause low compression also.

 

[FishDuck]

Cylinder bore measurements are listed in the one photo, nominally a 0.020" overbore I believe and no obvious tape issues.

Piston skirt measurement was 4.392", so all of that plus the STD marking seems like everything is matching up that undersized pistons were installed at some point. Looks like this site offers 0.020" overbore pistons in a rebuild kit.