Knurling: A question for you machinist types
 

I've heard of machinists knurling the hole where a bearing sits. I assume this is to tighten up the fit. I'm thinking specifically of the bearings in a 915 transmission case that have pounded themselves loose in the case.
Is this an operation that my "local machinist" could perform to restore the press fit of these bearings in a 915 case?
-Chris

BTW: Hey you 3.6 and Turbo guys, esp those who "haven't had any problems", betcha your bearings are already loose in the case.

How loose is "loose"? Usually knurling is a lathe operation, and this would have to be done on a vertical milling machine if you were doing it to the case. If knurling would tighten it up, you might try Loctite 600, which is a cylindrical bonding adhesive (green in color yet runny like the blue 242 stuff for threads). That might do the trick, but take extra care to keep it out of the bearing. You will need a torch to break the bond again from the case...I guarantee you won't pound it out.

Not sure if this helps but something to think about.

-BG

Chris,
The best way to fix this is to machine out the hole to a specific size and make a sleeve with the ID for the bearing and the OD for the new hole you made in the original part. This way you can make a proper press fit without having to return to the scene of the crime a few months later. Does this make sense?? Knurling is a rather temporary fix, especially when high stress is involved.

Cheers, James

I've been a machinist (non automotive) for ten years and this is not something I'd recommend. Knurling is typically done on a lathe so I don't think you could do anything with your tranny case. I've tried holding knutling wheels in a fixture on a mill but flex always seems to be an issue and the results are never satisfactory. Knurling at best is a band-aid approach to problem solving.

But who knows, maybe it's common and I've just never seen it done. Someone else want to chime in?

Good Luck,
Jim

Thanks BG,
I use Green Loctite now but I'm aware of one instance where 915 bearing races loosening up after it was used.
The problem (I'm told) is that the bearings have a press fit in the case but when the case heats up and expands, the press fit either goes away or the becomes less secure allowing the races to pound themselves loose in the case.
-Chris

Caught ya!
I was trolling for you James. :D That answers my question.
Thanks everyone.
Chris

Chris, The specs on the case should be able to tell you how much of a press fit you need. A common problem is that the specs are in Metric and many times machinists in this country will round it to an inch size. They may do this but they must carry the decimal point 4 places. Still this can be a problem. If you are .0002" over on a hole this can cause the situation you describe. Ze Germans are very precise for a reason, especially when it comes to the coeffiecent of expansion for dissimilar materials etc.

Cheers, James

Another possibility is to go to an oversize bearing.
Some makers have a selection of oversize ball bearings...where the o/d is +++
or the i/d is - - - .
This can save a piece of equipment sometimes.
If you do use this method....make sure the case is marked as such...for the next guy..or you down the road.
Bob

some cases get so bad that you can move the bearing race sideways, and easily see the slop. those cases should be trashed. a lot of races come out when the case is heated in the washing machine, but still fit kind of snug, cold. those can be set with red loctite. it would be real hard to knurl the inside of a hole, and like previously expressed, it doesn't last. the high areas just get pounded down again.

These bearing bores always wear egg-shaped, in whichever direction the gear pair is thrusting apart. If it's so loose that the bearing feels loose, then it's way beyond knurling or loctite, and needs to be sleeved.

When sleeving, it's difficult to find the original center without a centering jig, or a NEW matching housing to temporarily bolt on for centering in the Bridgeport.

New interference fit should be about .0015" (one and a half thousandths).

When we rebuilt my 915 a couple of years ago we "peened" the case and then bonded the bearing race in with red loctite. The peening gives the red loctite something more to grab onto.

Two decent fixes.
1) weld it up and recut/bore
2) interference fit insert and recut/bore

A tool and mold shop should be very familiar with these procedures.
good luck

I'm resurrecting this thread because I measured a 915 case and found that the hole in the middle case for the mainshaft bearing was pounded oval by .0035". But get this, it was oval side to side, not up and down as I would expect from getting pushed away from gear thrust against the pinion shaft gears.
Weird.
-Chris

Another option:
have the bore opened up .010" and then hard chromed and ground to size.
It aint cheap but it works well.
I'd bet you can find a good used case easier than repairing it, but if it is something special it might be worth it to fix.
Knurling is one step above bubble gum.

I'm ( somewhat ) familiar with hard chrome.
How do you do that to an aluminum case? I thought this only works on ferrous metal parts.
---Wil

You hard chrome the bearing race. But why not just have it professionally resleeved?

Alot of options were presented here...You can also swedge the material around the bore with a chisel and bore back to size, but this type of fix is only a temporary solution, and of the "Mickey Mouse" nature. I dont know what type of mat'l the cases are, but the suggestion of chroming the bores and grinding to size, is another option, but yet not the proper fix.
The bearing bore in question should be bored oversize, welded up and bored back to size. Press in new bearing. Reassemble. Drive car.
Simple. :-)

Chris, I just got off the phone with my bearing vendor. He recommended a product from either Permatex or loctite called "Press Fit Repair". It is a thicker bodied loctite and manufacturers are boring out their bearing housings slightly oversized and using the product so that the housings are not stressed out. He said they have had excellent results with it.

Hope this helps, Aaron.

Bore it larger (well oversized), sleeve it- have a sleeve made, and press it back together or tig weld the sleeve to the case...

Please try to stay away from the Loctite / Permatex fixes where bearings are concerned; they are only temporary.
As suggested earlier, try locating a slightly oversized bearing with the same size bore. Have the case bearing journal bored to the proper press fit size and insert the bearing.