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Crank Rebuilding Issues


I'm currently home schooling myself on crank rebuilding, so starting a thread on this topic may generate some useful information. A concern has arisen regarding labyrinth seal o-ring size. I noted that most 350 lab seals are using too large an o-ring cross section. Engineering specifications for the 0.5 x 4.0 mm lab grooves require a 3.0 to 3.1 cross section diameter o-ring while I at first ordered a 3.5 size before looking closely. I'm pitching those Viton o-rings and now have gone to the 3.0 (3.1 may be best but are limited if available sizes). I could be mistaken so I am throwing this out there. But if right, I believe this is a critical consideration.

Also, does anyone know what the acceptible limit for a used lab seal I.D. is? My best used one is 30.08 mm which does not wobble much on the shaft.

- Bob

              I believe the lab seal uses a 'fat' Oring for good reason. Have 'best measured' the groove at 3.9mm wide by 2.7mm deep.
       The lab seal may not be an interference fit in the case, it may even be a loose fit (unable to check) to allow for expansion of the alloy. The Oring obviously provides a pressure gas seal but it may also help to stop seal rotation at low case temperatures. I would be reluctant to reduce its cross section diameter.

      I would guesstimate the seal was designed with 0.05 clearance ( 2thou or 1 thou all round) from center shaft. Would be happy to use one at 30.08mm dia. Maybe not if clearance rose to 3 or 4 thou all round, IE 30.20mm diam. New seals can be wobbled 'slightly' on the shaft, they run with metal to metal clearance as above. Maybe our racer friends can better comment on wear limits.                                                                                                         
                                 Brian.        Did write a more comprehensive reply but hit the 'lose' button on keyboard just before posting (not the first time) above is a short summary  :'( . (Site would benefit from a 'draft' post section that automatically saves. like email, is this possible ? )

My concerns are based on the following:
1. All metric o-ring specification charts Ive seen indicate that a 3.5 mm cross section is too large for the lab seal groove.
2. The original o-ring, as best as I can measure (even considering deformity), is no more than 3.2 mm dia.
3. A recent comment I read on this site talks about the crankshaft rocking due to the large seal's outer diameter. That seems to be too much seal outer diameter if it supports the heavy crankshaft weight in that way.

I bolted a used lab seal with a 3.0 mm o-ring between two case halves and manually checked the fit. It was too tight to easily dislodge, i.e. it was forming at least a light press fit. I believe this would provide a good gas seal. The seal and case are both aluminum so differential expansion should not be a big issue.

Just say'n.

        You quote the groove size as 0.5 x 4.0mm, is the 0.5 a typo. As best I can measure on a built up crank, groove is 3.9mm wide (4mm drill shank doesn't quite fit) by 2.7mm deep (using electronic depth gauge, readings between 2.6 a nd 2.8mm). That gives a cross section area of 10.53mm2.
     Taking your concerns in order.
          Have never seen an Oring spec chart so can't really comment but accept what you say. A 3.5mm Oring has a CSA of 9.6mm2 so in theory would compress into the groove. The oring on my used crank has been highly compressed and almost fills the groove.
          Don't have an unused original Oring so accept your measurement at 3.2mm, this should easily compress into the groove.
          I think that was my comment. The crank only rocks when placed in a single case half due to sitting high on the Oring. I now don't see this as an issue, once the top case goes on the Oring is compressed, all the forces even out and the crank bearings seat correctly. As a matter of interest, as a precaution, I had my 45 year old NOS Rockford rebuilt crank trued up by SEP in the UK. They commented the crank sat high in the bottom case due to the uncompressed Oring, obviously something they were not used to.

   Could you try bolting up the seal without the oring and see if is clamped tight. As far as I can tell the seal was machined to a smaller diameter than the adjacent bearings possibly to allow for greater expansion of the alloy compared to the steel bearings. This would help confirm if the seal relies on the Oring to hold it firmly in place and stop any rotation. The inner crank bearings are an interference fit in the cases but they also have a knock pin, the seal doesn't

  With the high degree of compression applied to the original BS oring, I believe there is no possibility of any gas leakage past the outside of the seal and will never apply sealant, simply not required.

   Hope this helps,   Brian.


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