RMS (Rear Main Seal) install....too deep? 76 R75/7

[skittlz74]

So I replaced my RMS in my 76 R75/7 with the cycle works tool. BUT I didn’t use the washers under the bolts...careless oversight. The RMS is seated a mm or so under the lip of the case and not flush. Will this leak? Should I pull the seal out and start over? Please advise. I already have the flywheel back on and the clutch.....

 

[daz]

Compare answers to chewy1219 on Boxerworks.

 

[20774]

Here's a few older threads for reading that discusses the rear main seal (RMS):

https://forums.bmwmoa.org/showthread.php?53926-Rear-main-seal-installation-questions

https://forums.bmwmoa.org/showthread.php?94754-Removing-airhead-rear-main-seal-made-easy-(pictures)

 

[skittlz74]

Compare answers to chewy1219 on Boxerworks.

Yeah that is me also. Just looking for a solid answer as I have read a ton of threads but nothing really states that this depth is bad or good or acceptable.

 

[20774]

Matt Parkhouse discusses replacing the rear main seal in the December 2011 ON...it is in the archive section on the main MOA website. He doesn't give specifics but does mention using various tools to get it installed. He says the tool brings it 95% in place and then he taps is home. Seems like the issue is where the ID of the seal is going to run on the crankshaft. If it runs in the old rough area of the crank, that would wear the seal out faster. Did you end up smoothing out the surface where the seal rides on the crank?

Oak has also mention rear seal replacement in numerous Airhead Airmails over the years.

 

[gtgt_bangbang]

rear seal is seated a mm or so under the lip of the case and not flush. .

below is pic of of a used crankshaft tail-end

The seal landing is the rear-most journal step, a uniform cylinder for something under 10mm width ,
and you can make out out slightly polished band where the old seal rode.

I got no reference to anything else at this moment (case bore, inside interferences , ect) that could complicate/ interfere in the area.

I dont see why 1 mm deeper seal seating would hurt , at least according to the seal journal,
and you might have done yourself a good deed by moving seal further ~ past the old seal/journal interface point

bcos even if there is a hint of wear (a mere polished strip ), and an actual groove is a whole nother level of problem ,
a new seal will break-in better on fresh interface ( er, 50 years old & still fresh metal from cylinder grinder doing crank originally)
A new seal riding on a (old seal's) polished ring will not seal as well as fresh metal mating to fresh seal lip.



veering off your Q? but not UN-related.

If I recall, airhead cranks are surface-harded after final grind, so regrinding for oversize bearings ( without re-treating) is a nono?

A trick to restore to A) a non-mirror surface (remove old seal polish) or to B) do a final 'polish' (at shell bearing journals)
on every crank I've installed (whether re-installed as-removed , or back from a grinders) is to

* wrap a strip of v fine SiC paper around the journal (quality paper, 1500 ~ 2500 fine),
- the strip width is same as journal width,
- the strip length wraps around journal, almost but just under 2X, double layer, but avoiding a raised "hard" spot in the wrap.

* wrap a flat-type shoestring flatly & 1 layer, spiraled around full width of journal, aglets upmost , to grasp string ends.

* then lightly pulling the shoestring ends up & ..up? (I be like ) dragging the paper along, just a few times is plenty ,
Once you get down the proper string tension and syncopated foot shuffle, the paper will spin easily around the journal, polishing it.

- of course you throughly wetted the SiC paper & all around with kerosene/ect lubricant during wrapping .

- Breaking-in the paper beforhand, with a little flat rubout can also clean & tame the sharp new paper a bit, not a bad idea...
I have some flat v.fine diamond plates that can clean SiC paper, paper sitting grits-up on flat glass, but whatever

The goal is to uniformly take off any tiny non-uniformities , embedded grit, ect , and to give a bearing surface just a slightly finer finish than the final stone could, without removing a measurable amount of metal.


THe last thing I ownder; does there happen to be a " journal restorer sleeve " for really bad seal journals
( for instance https://www.skf.com/us/products/ind...smission-seals/wear-sleeves/skf-speedi-sleeve)
of the correct diameter & depth to restore a highly worn seal journal on the airhead crank ...

I have a speedisleeve for final drive INPUT gear ( rides in that green seal ) that I'll be trying for my FD,
but the crank of course is a different animal

 

[skittlz74]

below is pic of of a used crankshaft tail-end

The seal landing is the rear-most journal step, a uniform cylinder for something under 10mm width ,
and you can make out out slightly polished band where the old seal rode.

I got no reference to anything else at this moment (case bore, inside interferences , ect) that could complicate/ interfere in the area.

I dont see why 1 mm deeper seal seating would hurt , at least according to the seal journal,
and you might have done yourself a good deed by moving seal further ~ past the old seal/journal interface point

bcos even if there is a hint of wear (a mere polished strip ), and an actual groove is a whole nother level of problem ,
a new seal will break-in better on fresh interface ( er, 50 years old & still fresh metal from cylinder grinder doing crank originally)
A new seal riding on a (old seal's) polished ring will not seal as well as fresh metal mating to fresh seal lip.

View attachment 89661

veering off your Q? but not UN-related.

If I recall, airhead cranks are surface-harded after final grind, so regrinding for oversize bearings ( without re-treating) is a nono?

A trick to restore to A) a non-mirror surface (remove old seal polish) or to B) do a final 'polish' (at shell bearing journals)
on every crank I've installed (whether re-installed as-removed , or back from a grinders) is to

* wrap a strip of v fine SiC paper around the journal (quality paper, 1500 ~ 2500 fine),
- the strip width is same as journal width,
- the strip length wraps around journal, almost but just under 2X, double layer, but avoiding a raised "hard" spot in the wrap.

* wrap a flat-type shoestring flatly & 1 layer, spiraled around full width of journal, aglets upmost , to grasp string ends.

* then lightly pulling the shoestring ends up & ..up? (I be like ) dragging the paper along, just a few times is plenty ,
Once you get down the proper string tension and syncopated foot shuffle, the paper will spin easily around the journal, polishing it.

- of course you throughly wetted the SiC paper & all around with kerosene/ect lubricant during wrapping .

- Breaking-in the paper beforhand, with a little flat rubout can also clean & tame the sharp new paper a bit, not a bad idea...
I have some flat v.fine diamond plates that can clean SiC paper, paper sitting grits-up on flat glass, but whatever

The goal is to uniformly take off any tiny non-uniformities , embedded grit, ect , and to give a bearing surface just a slightly finer finish than the final stone could, without removing a measurable amount of metal.


THe last thing I ownder; does there happen to be a " journal restorer sleeve " for really bad seal journals
( for instance https://www.skf.com/us/products/ind...smission-seals/wear-sleeves/skf-speedi-sleeve)
of the correct diameter & depth to restore a highly worn seal journal on the airhead crank ...

I have a speedisleeve for final drive INPUT gear ( rides in that green seal ) that I'll be trying for my FD,
but the crank of course is a different animal

Awesome!! Thank you for the very detailed response; I am going to go ahead and button her all up and try it out tomorrow!!

 

[gtgt_bangbang]

button her all up and try it out tomorrow!!

YVW !
only appreciation I'd ask is feel free to deleting / editing down 99% of my repky & pix from the

box

like I did of yores

anybody that dont get it the first time , dont need a 2nd retelling & I for one dont mind the efficient use of thread

God Speed and Good Mileage to you!

 

[skittlz74]

UPDATE....yes, 1mm is enough to make the RMS leak, and quite substantially. Tore back down, installed another seal with the tool from Boxer2Valve, buttoned back up and she is now as intended

 

[kbasa]

An "aglet" is that little thing on the end of your laces, if you were wondering. Excellent word, OP.

 

[gtgt_bangbang]

UPDATE....yes, 1mm is enough to make the RMS leak, and quite substantially. )

appreciate your current & future updates.
Did you experience that leak or read about it the possibility ?

I confess its admit its hard to estimate jobs without the parts laid out,


but that seal journal IS uniform for about 2X of the seal depth, all OTHER things being equal & benign, that joint should work with an 0.040" depth variation , beneath a " flush" seal seating.

if there are other parts to assembly ( case bore, ect) which may leak, I would have no clue until my next airhead

Although after several l 4/6/8 rebuilds, which have rear seal totally similar to the 247, I dont usually see early rear end leaks ( on the engines, any way ...



anyway , IMHO, its far better to wrench safe & redo early in the process at the expense of a couple expendable parts, than to notice it roadside far from home



hey, just a question ; do you plan to center up the pressure plate ( reduce clutch assembly run-out as much as possible) to make a smoother running engine?

When you torque down the clutch, the assembly can have run out a significant amount, cause an imbalance.

I had to carefully smack out ~ 0.020~030 run out between flywheel & compression wheel 21 21 1 231 666