Farmall A Compression

Ah...a loaded question for sure.

I've written here a few times about my poor Farmall A that just plain runs uneven. I really don't want to address that right now as I've done it already and I have a plan of attack to try and find the culprit.

None the less, while I'm waiting for my manifold gaskets to arrive I thought I would ask about compression.

Part of my trouble shooting I checked the compression. Each hole gave me a reading of 93/94 PSI warm.

This is on a gas tractor that has had it's head rebuilt probably about 30 hours ago. I also installed new rings at the the time. I honestly didn't know what I was doing, I was just learning about engines at the time so it all seemed like a good idea. The liners didn't look all that bad but today, knowing that for a few more dollars and a bit more labor the liners would have been replaced too.

According to data I found on Webster's site 102 PSI is speced for the C113 gas engine. I haven't a clue if that is suppose to be a hot reading or a cold one. Mine did read a bit lower cold.

With new rings, even in old liners does 93PSI seem a bit low or about right. It doesn't smoke, at least enough that's noticeable. When it was running well it drove my woods mower deck just fine.

Thanks,

K

Well with new rings but old sleeves that reading is probably about right due to sleeve wear. The compression reading you read is probably for a new engine as in new sleeves and matching rings. I will almost bet that the engine in my BA which is half B and half A has around that is not a bit less since I used all used parts to build that engine

Several variables are in the mix of compression pressure. The following list is not complete.
Speed of cranking
Throttle wide open
Number of strokes for reading
Accuracy of gauge***
Warm or cold
Run recently (weeks) or not months/years
method of gauge attachment to hole

Your pressures are for sure in alignment with expected pressures on that engine. and close to one another. I would say no issue in those readings. Jim

*** gauges can be 20% different from perfect actual pressure, including both cheap and expensive gauges. Mechanics check their gauges against a master gauge periodically. Jim

Thanks for the insight guys.

My gut feeling was that 93/94 PSI wasn't bad, it doesn't hurt to ask though.

I also never really thought it was the reason why my tractor was running so poorly. I'm pretty sure my BN, with it's very worn engine is in the 80 PSI wise and runs like a top. Maybe not as powerful a top as it was when it was new but you can bet on it starting.

The number is less important then the fact that they're all even in the end.

K

i also agree that dry, warmed comp of mid 90's is likely way fine.

Thats a good reading. Do all the plugs look the same as that can tell a lot next would be wires and do you have a mag or a dist as worn bushings in a dist will cause problems. Just put a timing lite on each wire and watch for steady fire. Check each wire that way and if the dist or wires are bad it will show with intermitten fire.

Compression pressure depends on a lot of variables including head style, piston head design as well as wear. A C113 engine to have 102psi would have to have a 6.9 compression ratio which is possible for a factory original gas C113; i.e step head pistons with a gas head. That is 100% efficiency for a 6.9 CR. To estimate what a compression test reading should be, multiply the compression ratio for the engine as equipped (piston design and head)by 14.7 psi which is one atmosphere pressure and then multiply by about 90%. If I remember correctly, the old distillate engines had compression ratios less than 5 to 1. That would put the expected compression reading to be below 70 psi and that engine will run like a top.