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#402615 - 11/08/11 4:09 am How to replace bushing in pre-unit gearbox?  
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Peter Quick Online content
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I'm replacing the brass? bushing inside the 4th gear sleeve on the mainshaft. Gear box is a STD pre-unit swing arm box. It was leaking oil around the main shaft. Getting the old bushing out was a bit of a pain but now I'm confronted with a tolerance question. The inner diameter of the replacement bush is a perfect fit on the mainshaft. Tight but still smooth rotation. However I measured the outer diameter of the bushing and the diameter of the inside of the sleeve gear. My bushing is about .004 to .0045 larger than where it has to be pressed into. I know there needs to be some interference for a tight fit but before I press this in (I'll warm the gear first) I wonder if it isn't a bit too big and will mess up the nice fit I presently have on the inner diameter of the bushing? What should an ideal interference fit be to not cause much distortion?

Peter


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#402641 - 11/08/11 7:46 am Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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DMadigan Online content
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ca, us
Almost any press fit that keeps the bush tight in the high gear will shrink the inner diameter. You have to ream the bore after installing to the proper clearance. The bush should be bronze or Oilite, not brass.
I just did this on an A65 using off the shelf Oilite bushes from McMaster. I am not sure of the original part cost but I doubt they are cheaper. The bushes even have an X groove inside.

Last edited by DMadigan; 11/08/11 7:46 am. Reason: sp
#402642 - 11/08/11 8:58 am Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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triton thrasher Online content
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Some of us wouldn't attempt this job just because of a small leak. Sorry that's not much help, but good luck.


Amateur Loctite enthusiast.
#402669 - 11/08/11 2:30 pm Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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Alex Online content
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Yup, it's pretty much a given that you will have to ream it. Honing is even better.


A smattering:
'53 Gold Flash
'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
'37 Rudge Special
sixtyseventy Lightboltrocket road racer...and many more.
#402686 - 11/08/11 2:40 pm Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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Peter Quick Online content
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Looking at the bushing I bought from one of the usual suppliers I think it looks like brass. It is pretty soft stuff. Was bronze or Oilite ever used as original in this application? I know the layshaft "bearings" looks to be bronze but inside the 4th gear sleeve gear is this necessary?


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#402712 - 11/08/11 7:03 pm Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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DMadigan Online content
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Not if you stay in fourth gear! Otherwise the mainshaft is turning in the high gear. The only lubrication is the small amount of oil that geta past the first bush.

#402728 - 11/08/11 7:02 pm Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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Ignoramus Online content
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There is no way it would be straight brass. It takes a lot of load supporting the mainshaft clutch and gearbox sprocket (the ball bearing clearance will still allow the pinion to move a lot if the bush is in bad condition). Straight brass just wouldnt do it.

You should aim for an interferance fit in the OD of the bush of no more than .001" or you will have all sorts of problems pressing it in. 004 will be way to tight. But that all depends on what equipment you have to press it with. 004 will go in but will colapse the id too much and realy will take loads of pressure.

what i do is relive the first 3/4 inch or so of the OD to only just larger than the hole in the gear (like a tight hand press in) so that you get it started nice and square then more of a press required after that.....dont forget to re drill the holes in the gear.

There is penty of choice regarding material ie bronze, but brass isnt one of them, there are loads of bronzes that will do the job.

it needs sizing on the ID afterwards like Alex said above. This can also be done with good results if you take your time and hand lap it with some 240 paper, hence the need to keep the colapse of the ID to a minimim, no way it will move with only 001 press fit. You are looking for around 002 clearance on the shaft.

If the diameters of the shafts are the same as the M series box the ID of the bush should be exactly 7/8" or 22.22 and the shaft will be factory ground to give optimim clearance on that, no need to reinvent the wheel.

heating the gear wont make a lot of diffrence on that diameter.

If you are worried use some red loctite on the od of the bush.


Last edited by Ignoramus; 11/08/11 10:47 pm.

"There's the way it ought to be and there's the way it is" (Sgt Barnes)
#402772 - 11/09/11 12:35 am Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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My first question is what you are doing your measuring with,if using a dial caliper they easily can be off .003 or so going from ID to OD. As a fairly advanced home machinist there is no way I would try to make a bushing using a dial caliper to do the size measuring. And in that size a .001 interference fit is plenty. Sizing after installation is also normal.


John
#402791 - 11/09/11 1:39 am Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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Ignoramus Online content
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JohnEvans recons:
"My first question is what you are doing your measuring with,if using a dial caliper they easily can be off .003 or so going from ID to OD"

Good point John....I guess ive been in engineering to damm long and just kind of assume appropriate measuring gear.


"There's the way it ought to be and there's the way it is" (Sgt Barnes)
#402855 - 11/09/11 2:19 pm Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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Mr Mike Offline
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The machinery handbook provides all you will ever need on fits. You don't want it so tight that you exceed the yield of the material. You also have to consider your measurement tolerances and manufacturere tolerances which you probably don't know. A good machine shop with a small press can do this and get you a nice sliding fit for the shaft.

For a samll leak on a BSA...well I dunno. When you look under even a well assembled BSA it is rarely dry. Between primary gasket seals, blowby, case seams in trans and engine, shaft bushes, and breathing ports etc it is an exercise in futility to completely seal these old bikes up. Many have tried and few were ever seccessful.

Mr Mike

Last edited by Mr Mike; 11/09/11 1:26 pm.
#402894 - 11/09/11 6:13 pm Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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Well you are all right about my so-so measuring device. Undaunted I cleaned up the inside of the gear with some fine sandpaper and worked on the outer diameter of the bushing a bit. I pressed it in and it had shrunk a bit but not too bad. I went over the inside a few times with a small brake hone until the fit was a nice tight and smoothly sliding onto the mainshaft fit.

This was just as much about if and how to do the job as fixing a leak. I didn't say how minor it was however. It oozed a good bit. Also this nice STD gearbox was not in anything as I had swapped it with a non leaking gear box, so I knew that the leak could be fixed as I have another that doesn't leak at that place. The biggest pain was getting the old bush out and lacking the knowledge about the various aspects of the job in advance. It has all worked out well. Peter


check out: www.bsaunitsingles.com
2500 BSA part numbers with inventory in stock just for the unit singles!
#403856 - 11/15/11 4:44 am Re: How to replace bushing in pre-unit gearbox? [Re: Peter Quick]  
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My experience...take the shaft and the gear and the new bushing to a machine shop and let them do it. Pay the $60 and use their knowledge. It will save you many grey hairs.

Also Frank Diehl has an excellent oil seal holder that replaces the big oil seal and the threaded ring shaped nut. This really works and is worth every penny of $75.


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