farmall cub engine

dont know if this is a dumb question but...
i am a fifteen year old that wants to learn to rebuilt engines tractors etc. and i have a farmall cub (pretty sure 1949) i am in the middle of rebuilding. If i were to install dome top pistons (for increased horse power), would that make it origional or not.

Also, what should i do if the first three cylinders have 105-110 psi and the fourth has 40 psi? Bad valves? bad bore?

Hi, I don't think dome pistons are available for that
motor. Low compression could be broken rings on
that cyl. Ed will

Hi Elijah,
I can't answer your question about the pistons, but here's something to help with the low compression:
Take all the plugs out and unplug the coil wire.
Open throttle all the way.
Run your compression check again.
Now, squirt about a tablespoon of oil in the low compression spark plug hole.
Run a compression check in that cylinder again.
If the compression stays the same, it would be an issue with the valves, (top end).
If the compression increases, it's either rings or piston. (Lower end).

By putting oil in the cylinder, you've temporarily "sealed" the rings and that's why the compression would increase. This is not a fix, just a way to troubleshoot the problem.

Hope this helps.

I'll guess valve not seating properly. It may just have carbon stuck under it seeing as all the others are so good.

Welcome Elijah! Your questions are welcome here, hope we can be of help. Good to have young people
interested in preserving the past, us old farts won't be able to do this forever! LOL

My advise would be for you to buy a shop manual for the tractor. Read it several times, even the part
you're not working on. It will give you an idea how things were done back then, how equipment was built to
be field serviceable with basic tools and abilities. It will pay for itself many times over in mistakes not
made!

Since you are already in the process of rebuilding the engine, don't worry about why the compression was
down on one cylinder. When you get into it, the problem will be obvious, and will be corrected if
everything is done properly.

As for putting in high compression pistons, DON'T! First, you won't find them, will have to have them made,
$$$$$ and time wasted! This is your first engine, keep it simple, put it back exactly as it is supposed to
be, be proud of your work when everything runs as it should. Changing compression can cause all kinds of
problems, don't want to get in so deep you get discouraged and never get it right.

Save the performance work for later, and do it to something that has potential for performance that cheap
parts are available for, not an antique tractor!

Keep us informed. Take before and after pictures. Let us know how it's going, don't be shy about asking for
help, you'll get 'er done!

That would be valves i would check the valve lash before tearing down the engine. The other three have good compression so its unlikely that piston is bad maybe a simple valve job on that cycl will do the job.

Hi, I don't think dome pistons are available for that
motor. Low compression could be broken rings on
that cyl. Ed will

I thought the newer cubs had higher compression than the old ones and the pistons would fit. Maybe not. The newer cubs were 5 or 6 more HP.

Hi, I don't think dome pistons are available for that
motor. Low compression could be broken rings on
that cyl. Ed will

adjust your valves 1st then try compression check again
valves will wear and get too tight not seating leading to low compression
check out the link
I was 15 a long time ago with a cub, cub is still around where I grew up at
cub

so today i performed 2 compression tests (after the one i did earlier in the week) the first was after i put a tablespoon of oil in each hole. it read 1st-100, 2nd-100, 3rd-150, 4th-100.
the second read 1st-120, 2nd-130, 3rd-130, 4th-120. my grandfathers opinion is he would take it to a shop, i don't want to do that. he thinks the valves are not seated right or the rings
are sticking. what do you guys think?
thanks
Elijah