8N Transmission Bearing Question

[Noah H.]

Hello,
I am working on a 1948 Ford 8N that had a bad growl in the transmission. Upon splitting the tractor and inspecting things, I determined that the mainshaft pilot bearing (part number 9N7120) was the bad one. Its outer race is cut into the input shaft drive gear, and it presses onto the transmission mainshaft. It was easy to replace, although not cheap. I also replaced the input shaft bearing (part number 9N7066) to be safe.
The result of this is that the mainshaft bearing preload is incorrect, and attaching the input shaft to the transmission puts an enormous preload on the mainshaft and input shaft bearings. The shims used to set this preload on the mainshaft and input shaft bearings are installed from the back of the transmission, and require splitting the transmission from the differential housing to access. I'd like to have this tractor back together soon, and I could avoid splitting the transmission from the differential if I put the same style shims under the input shaft housing - it would have the same results inside the transmission. Is there any reason I shouldn't shim from the front of the transmission, under the input shaft housing, instead of from the back of the transmission? I did a little looking, and it seems that Sherman transmissions that mount in place of and act as the input shaft do use shims there to set preload. The only problem that occurs to me is that the input shaft would sit slightly further forward than before, meaning that the input shaft gear would mesh with the countershaft gear in a marginally different spot. I don't know enough about the properties of gears to know if this could cause any issues. It helps visualize the transmission to see a parts diagram or Just8N's YouTube video showing the way the transmission goes together.
Thoughts?
Thank you!
Noah H

 

[John0829]

Have you measured to know how much shim is need to get the proper preload? .010 to .020 or significantly more?

 

[Noah H.]

Have you measured to know how much shim is need to get the proper preload? .010 to .020 or significantly more?

I haven’t measured it, but looking at it it can’t be more than .025.

 

[John0829]

It is your tractor and if that is what you want to try then go for it. If the needed clearance was small, like the difference between a thin and thick gasket I would consider it safe, however adding shims will change where the 7120 bearing runs in the input shaft and depending on wear inside that shaft? If IRC there is no race for that bearing other than where it rides inside of the input shaft. Another thing, shims will push the input shaft forward deeper into the clutch plate and pilot bearing.

Been to long since I was in one of these trannies or an 8N for that matter but it is not that much work to separate the rear housing from the trans to do it right. Just my $0.02 if we still can use pennies.

 

[Noah H.]

It is your tractor and if that is what you want to try then go for it. If the needed clearance was small, like the difference between a thin and thick gasket I would consider it safe, however adding shims will change where the 7120 bearing runs in the input shaft and depending on wear inside that shaft? If IRC there is no race for that bearing other than where it rides inside of the input shaft. Another thing, shims will push the input shaft forward deeper into the clutch plate and pilot bearing.

Been to long since I was in one of these trannies or an 8N for that matter but it is not that much work to separate the rear housing from the trans to do it right. Just my $0.02 if we still can use pennies.

Thank you John for your input and assistance. I've done some considering, and I think I'll risk the shims, as I just can't afford to split the tractor right now. During reassembly, the issue you mentioned became apparent - the irreplaceable race in which the 9N7120 front mainshaft bearing rides is bad as well. On eBay, I can get a used one in good shape with a race free of pitting or any other noticeable damage. It costs a third as much as a new input shaft. However, what you hear is to always replace bearings and races together, and if I bought the eBay input shaft, I would be mixing new parts with (good) used parts. I know forking over the cash for a brand new input shaft is the best choice, but in your or anyone else's opinions, could I get away with the good used input shaft? Or would the bearing fail soon?
BTW this transmission doesn't have to be perfect because I know the countershaft bearings have wear, but I'd still like a quality fix on the input shaft and front mainshaft bearings.

 

[John0829]

As long as the new one is not pitted just replacing the 7120 bearing will be fine. You said you want a quality fix but at best you are doing a patch that I doubt will not cure your noise. Replacing the bearings at the front of the input shaft still leaves the rest of the old bearings in place and since you determined the 7120 was bad plus you have pitting in the race that means there has been water in there. You can rest assured the other bearings are probably on the way out as well. As for not being able to afford splitting the tractor, all that costs is some time, some gaskets and you are already halfway there anyway.

It is your tractor do as you want but if it were me I would fix it now instead of putting it back together only to discover it has to be redone. Again just my $0.02

 

[careyo63]

My 2000 front bearing had a problem! NTM. The shop that split it mentioned it had a lot noise and more or less a common problem. luck have it all the Drive shaft nuts and bolts to a front mounted Hyd. pump was loose. 2piece shaft!! Hard to get access to them. After all the fluids replaced and sure enough very little to almost no noise now. IHO. Fix it and don't risk damage to anything else. Not that much to it I believe.