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Old 06-03-2012, 09:13 PM   #1
DiabloADV OP
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Measuring gearbox end-play

I spent 3 hours Saturday trying to get the shim measurements right after an overhaul of my 5sp gearbox. We had the right plates and a handful of depth measuring gadgets. I (and about everyone else there) kept getting different measurements. The box would get closed up, and be too tight. Then, do it again and the output shaft shim (which is visible before the seal goes on) would be loose. One guy had been at it for 4 hours by that time. I finally gave up and had to get home. I think using those indicators takes more skill than most of us at the tech day possessed.

Today, I tried a method I'd read about. Place little pieces of hollow-core solder so the ends sit across the outer races. Heat the cover and install and torque. Then take the cover back off. Measure the squashed solder, subtract 0.05mm, and you have your shim specs.



It worked perfect, and took 15 minutes instead of the tedious afternoon of ultimate failure. Plus you don't need a $70 flat plate and a $100 indicator. Or much skill.
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Old 06-03-2012, 09:16 PM   #2
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Also...here's why you don't use the kick starter on a '74 90/6...



Those gear nubs are almost completely worn through.
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Old 06-04-2012, 06:11 AM   #3
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Thank you.
It seems to make perfect sense.
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Old 06-04-2012, 06:13 AM   #4
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Wow!! Thats nuts!!! Thanks for the pics!


Quote:
Originally Posted by DiabloADV View Post
Also...here's why you don't use the kick starter on a '74 90/6...



Those gear nubs are almost completely worn through.
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Old 06-04-2012, 06:25 AM   #5
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Question Why subtract the 0.05 mm?

Being mathematically challenged, my first reaction when numbers are involved is to panic .


Is the .05mm a tolerance or...?
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Old 06-04-2012, 06:50 AM   #6
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Correct!

The thickness of the solder is the exact distance between the bearings and case. The .05mm (.002") is the clearance needed.
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Old 06-04-2012, 07:11 AM   #7
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Hmmmm. That's pretty slick. I've never rebuilt a trans before, at least not on an airhead, but now I kind of want to just try that trick. Cool.
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Old 06-04-2012, 07:40 AM   #8
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Quote:
Originally Posted by ozmoses View Post
Is the .05mm a tolerance or...?
Yup. That's what gap is left over so it's not tight and oil can circulate.
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Old 06-04-2012, 07:41 AM   #9
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And...I bet this would work on a crankshaft. Place the solder, torque up the flywheel (with the old bolts). Remove flywheel, measure solder, install proper thrust washer/shim and install flywheel with new bolts.
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1970 R60/5

'91 Bill Holland Steel w/Dura Ace[/SIZE]
'01 Z3 3.0 Coupe. The Clownshoe.
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Old 06-04-2012, 08:42 AM   #10
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I advise people to do that with pistons and valves all the time and I had never thought about doing it in the tranny. I work on them semi-regularly. I might try that next time. I always look up the specs but I think they go to zero. For my own nit picky self, I think they shift better shimmed to the bare minimum. IMO, with all the right stuff and a lot of experience the measurements are still a bit of a guess for measuring the bearing height. I measure them from the inside race but you are never quite sure if the gauge isn't rocking.

Flywheels? I will probably stick with a dial indicator but you could do it that way. It's real accurate and a real time saver with pistons. If everything is right you don't even have to take it back apart! I do it all through the spark plug hole. I think it's a much more solid measurement than clay.
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Old 06-04-2012, 09:23 AM   #11
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So this will give you a clearance of .002" at 'room temperature'. Has anyone ever reckoned that clearance at operating temperature? That is, at the temperature of the gearbox while running, the steel gearshaft will heat up and expand in length, the aluminum case will will heat up and expand in length, and the delta-length will be more fo the aluminum than for the steel (different "coefficient of expansion" of the metals, about 3x more for the aluminum than the steel). For example, the .002" cold clearance may be ~.009" hot.

Just curious. Been meaning to do the same figurin' for the crankshaft endplay. But if some (other) techno-geek has beat me to it, I'm no glory-hog...
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Bill Harris screwed with this post 06-04-2012 at 11:40 AM
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Old 06-04-2012, 09:57 AM   #12
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Quote:
Originally Posted by Bill Harris View Post
Has anyone ever reckoned that clearance at operating temperature?
Yes...and I know exactly who....

The engineers at BMW that decided that 0.05mm was the right number. They get paid to think about such things.
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1970 R60/5

'91 Bill Holland Steel w/Dura Ace[/SIZE]
'01 Z3 3.0 Coupe. The Clownshoe.
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Old 06-04-2012, 11:31 AM   #13
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Quote:
Originally Posted by DiabloADV View Post
Yes...and I know exactly who....

The engineers at BMW that decided that 0.05mm was the right number. They get paid to think about such things.

Are they the same guys that eliminated the output shaft circlip and then put it back on years later?
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Old 06-04-2012, 11:34 AM   #14
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Who knows? The guys who designed the transmission may have been long gone by then!
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Old 06-04-2012, 11:43 AM   #15
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Nah, I suspect that the old guys who designed the tranny retired and came back to the office for a visit, found out what the whippersnappers had done with the circlip and screamed "vat vur you tinkink???" and got it put back in.
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