Bronze gear bushing clearance - Alfa Romeo Bulletin Board & Forums
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post #1 of 70 (permalink) Old 09-17-2014, 08:37 PM Thread Starter
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Bronze gear bushing clearance

Whats consensus on dry bushing ID to shaft OD clearance? My old ones were between 3 and about 7 thousandths. The new 'clevite' type are pretty small, about 25-30 thou undersized and thus need to be boring barred- honed out. But I am wondering what is 'about right' Half a thou seems too small and 3 to 4 maybe a tad bold? was thinking 2 -ish ..??

thanks in advance bob
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post #2 of 70 (permalink) Old 09-29-2014, 10:43 AM Thread Starter
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I have got to say -- I have an entirely NEW RESPECT for RJ and anyone else who works on these 5 speed gearboxes. Naively I thought -- get the parts, do some light finish work on a few items, and it will go easily back together in an hour or two! Boy was I dreaming, I think the gearbox is more complex reassembly -- BY FAR-- than a motor. Even just getting the snap rings on is a hassle! Tolerances for bushings are hair thin and better be 'on the money' as far as perpendicularity is concerned. Anyway more to come as I continue to learn this art form.
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post #3 of 70 (permalink) Old 10-08-2014, 11:57 AM
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+1 on the respect for RJ. I went through the same "learning curve" when I replaced the gear bushings on my gearbox, and had to hone them out to the right tolerance. My third gear ended up being a bit tight and wanted to stick to the shaft the first time out once everything got warm. RJ kindly walked me through procedure to work the bushing in without a transmission teardown. Still working perfectly thousands of miles later.

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post #4 of 70 (permalink) Old 10-08-2014, 12:09 PM
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So what's the correct clearance and can the bushings be reamed to size with an adjustable reamer?
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post #5 of 70 (permalink) Old 10-08-2014, 12:43 PM Thread Starter
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As I understand it, there are two important pieces to this (again thanks RJ and let me know please if I got it wrong)
1. When you install the bushing, it has to be expanded in the gear hub so that it can expand no further.
2. Then, it has to be finished to the appropriate size (0.002 +0. -.0005) for the shaft

3. We discovered three things so far

a. When the bushing in the gear was correctly fitted at 1.5 thou over, and then the gear assembly completed (and strong snap ring installed) now all a sudden it starts to bind up. I would never have guessed that the snap ring would compress the hub at the end enough to take up half a thou-- but it did!
b. The finishing has to be absolutely perpendicular and straight-- no offset.
c. Once installed and all good, file down the bush to match the oil passages, and clean the snot out of it, using brake cleaner then detergent and water, followed by brake cleaner again. It took a good while to get all the black grinding residue out of the bushing.

Alphil unless you have a super supply of reamers, I doubt you will find one that is exactly correct for your shaft diameter + .002. I think the best that can be done is get close, then hone it carefully and slowly. I would not use an adjustable one, I doubt you can hold tolerance with it

Richard, did I miss anything critical or other important details? Many thanks
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Last edited by goats; 10-08-2014 at 12:46 PM.
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post #6 of 70 (permalink) Old 10-09-2014, 07:20 AM
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Goats,Thank you,this is very helpful.Do you expand the bushing by some mechanical means,with heat..?Would this work to expand the bushing; heat the gear first in a toaster oven,insert the bushing,heat "it" with a propane torch to expand it,let everything cool? AFA the adjustable reamer idea,I've never used one so had to ask.I have a worn out 1946 South Bend 9" lathe and I'm not a machinist so probably not a good combination for holding close,perpendicular tolerances.
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post #7 of 70 (permalink) Old 10-09-2014, 10:23 AM
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Quote:
Originally Posted by alphil View Post
Goats,Thank you,this is very helpful.Do you expand the bushing by some mechanical means,with heat..?Would this work to expand the bushing; heat the gear first in a toaster oven,insert the bushing,heat "it" with a propane torch to expand it,let everything cool? AFA the adjustable reamer idea,I've never used one so had to ask.I have a worn out 1946 South Bend 9" lathe and I'm not a machinist so probably not a good combination for holding close,perpendicular tolerances.
Generally, to assemble tight or interference parts, heat the outside part and chill the inside one. They should slip together easily and expand/shrink to a tight interference fit. Broach or lap the ID after that.

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post #8 of 70 (permalink) Old 10-12-2014, 05:17 PM Thread Starter
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Bushing clearances

I;ve gotten two very distinct recommendations on clearances --(inches)

SET 1 == 1st gear 0.008-.0.0012, 2nd and 3rd .005-.008

SET 2 -- from a technical manual "gearbox" although it says for 1750 model -- but I thought all the 5 speed gearboxes were the same from 1750 to 2000

1st = 0.0049-0.0067
2nd and 3rd - 0.0038-0.0055

When I took the trans apart, all were running about 6 thou, and the trans was fine. So set 1 seems a bit loosey goosey at least to me

here are some photos of where I am at. Would LOVE to get views on which set of clearances is correct! Obviously, clearance is important, but concentricity is crucial!

PS i used SET 2 as the guide for the clearances --- not set 1.
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post #9 of 70 (permalink) Old 10-13-2014, 06:43 AM
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Bushing

The reason it closed up when installed is the bushing was too long and needs to be shaved down so it isn`t longer than the gear`s thrust surface.

.003 is the limit I would want any bushing.

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post #10 of 70 (permalink) Old 10-13-2014, 09:11 PM Thread Starter
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Thanks Richard and 60sracer

I'm had a machine shop check my numbers and they measured 3rd at 3, 2nd at 4 and first at 3 thou- still I am going to re-do B (2nd gear) to get to 2.5 thou. Question -- I wont get binding when that mainshaft heats up and takes up a thou or two (I know the bushing and gear will expand a bit too, but just checking with the experts) --someone warned about that...

Next question relates to the three synchro hubs (tripods)-- is there any difference between these three hubs? Someone told me yes there was, but I can;t see it -- couldnt really measure it either unless its really, really subtle -- I dont have a good metrology setup . I could detect no real differences (more than 0.005 ish) ----???

60's racer thanks for the advice, thats how I will do the hubs and re-do second gear bush. Richard thank you for your expert advice. I will make sure I finish the bushing so it is not proud over the thrust surface. I think it was prooud by 5 to 8 thou or so, still enough to bind it up!
Let me know how I can help you guys with 24V !!!

bob

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Last edited by goats; 10-13-2014 at 09:14 PM.
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post #11 of 70 (permalink) Old 10-14-2014, 07:28 AM
Richard Jemison
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Tripods

The 5th gear tripod is different on 1 side.

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post #12 of 70 (permalink) Old 10-14-2014, 07:37 AM
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Quote:
Originally Posted by euro1750 View Post
+1 on the respect for RJ. I went through the same "learning curve" when I replaced the gear bushings on my gearbox, and had to hone them out to the right tolerance. My third gear ended up being a bit tight and wanted to stick to the shaft the first time out once everything got warm. RJ kindly walked me through procedure to work the bushing in without a transmission teardown. Still working perfectly thousands of miles later.
Euro1750,Can you elaborate on the honing method you used both while the gearbox was apart and after it was assembled? What was your initial clearance? Richard,5th gear tripod;different in what way?

Last edited by alphil; 10-14-2014 at 07:40 AM.
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post #13 of 70 (permalink) Old 10-14-2014, 07:54 AM
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I had a friend do the machining on the bushing, he used a spark erosion machine, we were concerned that a lathe tool might cause the bush to rotate in the gear (rookie observation, I have no experience machining anything like this).
Looking at my notes I have the clearance noted as 12 thou for 1st and 2nd gear, and 10 thou for 3rd. We were targeting 12 thou, but didn't think that we would be able to get 2 thou off with the spark erosion machine, so decided to move ahead, as the gear still spun on the shaft, albeit with a slightly sticky start (really quite minor). Now I am pretty sure that when I say 10 thou, I am talking about gear ID - shaft OD, so the diametrical difference, not the half clearance. Your numbers seem to be roughly half of what I am talking about.
As mentioned before, I did get binding when the box warmed up, and I assume it was the tight 3rd gear. I got lucky as the gear (or whatever was binding) worked itself in and did not have to pull it apart again, but it would seem I found the tolerance limit!!

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post #14 of 70 (permalink) Old 10-14-2014, 08:01 AM
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The initial clearances were not measured as the bushing (really only 1st gear) was shot. The shaft diameters I measures were:
1st : 34.87mm
2nd:34.87mm
3rd : 34.93mm
When I installed the new bushes the ID was smaller than the shaft (34.55, 34.35, 34.42) so clearly machining/reaming was required.

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post #15 of 70 (permalink) Old 10-14-2014, 01:06 PM
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just looked at your photo measurements, and you are indeed talking diametric clearance. For my rebuild I targeted .0012, and seems like I narrowly dodged a bullet with .0010 ( I am assuming that it was the "tight' third gear at .0010 that caused the binding...)
I would definitely wait for RJ to chime in, because .0005 does seem reasonable for standard bushing clearances, but there might be some other reason for a larger tolerance (thick oil, viscous damping, etc)
I think someone asked already, but what is the ID of a bush in a new gear?

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