8N Rebuild Ideas needed.

[ToadHill]

After not being able to start my 8N, I let it sit for a little over a year. I then checked the compression and had anywhere from 30 to 50 PSI so I decided it was time for a rebuild.

Yesterday after we removed the engine my youngest son disassembled it. I found the bearings in great condition, valves had no burn marks in them and they appear to have no blowby. Head gasket was in fine shape, and when I took the pistons out all the rings were in place with none cracked. Cylinder walls are not scratched or pitted, and no ridge at the top.

I'm at a loss as to what caused the compression loss. Anytime I've had a compression loss like that I always found burned valves or broken rings.

Also it looks like this engine was rebuilt once before as I found rods all marked 9N in an engine block marked 8N.

Any thoughts would be appreciated.

 

[tommu56]

I guess you didn't take a compression test with a couple of teaspoons of oil in the cylinders.

The oil would have sealed the rings to see if the top end was the problem .

tom

 

[CharlieS]

Lots of possibilities here, but a wet test would have definately been in order before dissasembly. Slight misadjustment of the valves could cause low compression yet have allowed a newly rebuilt engine to start.
If its been bored, undersize pistons could contribute. Couple that with excessive ring end gap or incorrect rings could cause problems.

 

[Diamondpilot]

These engines are very low compression but I am guessing you did a dry compression test. My feeling is if you would have put oil in the cylinders and cranked it over a little the compression would have been much higher.


Sounds like your son got a good lesson on how to take out a engine and put it back in.

Did you confirm valve movement and distributor movement?

Chris

 

[ToadHill]

Sorry I failed to mention it but the compression figures I gave you were with oil squirted in the cylinders. It was even worse dry.

The tractor had run fine for over 20 years. It started using a little oil in the year before I parked it, and the power was down, needed to downshift to go up hill in places that I never had to before. I didn't think it would be valve related (adjustment) since wear usually causes lash to increase not decrease.

 

[jrduty]

Had something similar happen with my '54 NAA. The 2 brass screws that hold the throttle plate to the shaft came off-never found them and it's an updraft carb- and after a carb rebuild the old boy was running great... for about 15 minutes. Started missing BADLY. Compression test(s) showed 0!! lbs. in one cylinder. I finally figured I'd holed a piston or something else bad and pulled the head. All 4 cylinder walls are beautiful, pistons look great, head needs a little attention but with 3500 hours it wasn't bad. And no sign of those 2 brass heads. I had the head rebuilt, put in new rings and rod bearings since it was right there and easy to do( the old bearings and the crank were in impressive condition), put it back together and it runs nice and strong like it always has. I'm guessing I had a valve stuck open?
Ya gotta love these old tractors:thumbsup:

 

[Soundguy]

I still think you jumped the gun... any engine like that that has set a while.. the rings can get sticky.. I would have soaked the cyls in atf for a few days, rotating it over by hand with the plugs out every day and adding more cheap jiffy store atf oil.

then you giver her a test.. pull start if you have to.. burn all that oil out.. warm her up, then do the dry/wet test and compair the numbers that will tell you rings or valves.

at this point you are left to measuring the cyl's, then guessing on what is good or bad.

if the cyls measured in spec I'd get new rings, new bearing inserts for mains and rods ( same as old size ).. ream the ridges, hand lap the alves and get a new pan and head gasket and crank seals and see what I got.

not a real good way to go.. but otherwise you are left to do a 100% rebuild with new liners and all without some good diagnostic work to point you in a direction to go.

soundguy

soundguy