Primary Cover Fitting Issue

Need some insight as to cause of issue with fitting the outer primary cover on my 1974 Commando 850.

I installed a new AN inner cover. No issues with installation.

After completion of clutch and alternator install I installed my old outer cover without the rubber o’ring and checked for clearances. No issues, dowels and center bolt lined-up perfectly, cover seated with no gap to inner cover. Rotation of engine and clutch showed no touch conditions.

Installed outer cover with AN o’ring installed on inner cover. O’ring fit into groove was good – held in place without any additional sealant. Installed outer cover, torqued center bolt to 25 lb-ft. There is a consistent .030 to.050 gap all around cover interface. Increased torque to 30 lb-ft. No change in gap. I let sit overnight in that state to see if any relaxation in joint (didn’t think there would be). Checked next morning – same gap, application of torque didn’t change gap.

O’ring is compliant (don’t know durometer) but in good shape (new). Ends where glued together match perfectly. Diameter is approximately .220”.

Inner cover cast-in o’ring groove is clean with nominal depth of .115”. Seems shallow for diameter of o’ring but not sure what design intent is.

Assuming parts are all correct – any suggestions?

Sorry... what is the problem?

maylar said:

Sorry... what is the problem?

Click to expand...

Problem: " There is a consistent .030 to.050 gap all around cover interface."

Are the covers designed to have a gap between them with the o'ring in place?

Mine's always had a gap between the cases. As I'm running a belt dry I've never worried about it. Assuming you're running with oil, does it leak?

Gadge said:

Mine's always had a gap between the cases. As I'm running a belt dry I've never worried about it. Assuming you're running with oil, does it leak?

Click to expand...

Yes. running with oil. No leaks but that is not how a surface o'ring seal should be designed. I know as I have been reminded several times on this site - the Commando was not designed - it evolved. However the alloy primary cover was new for the Commando so a little surprised.

Ok, I guess if that is normal I'll live with it - will probably reduce gap with retighten with engine hot.

Thanks

Assuming you have properly shimmed the new inner cover I see nothing wrong with the minor gap you describe. You don't want a metal to metal joint. Don't get too aggressive with the primary cover nut tension at risk of distorting the cover.

O rings despite being soft cannot be compressed to fit a space less than their total volume. All the engineering data for O ring grooves on rotating shafts has the O ring groove with a 10 to 15% greater volume than the O ring, the seal is achieved by the O ring ID being squashed but also deforming sideways but it never fills the groove. This application is fully static so the space is less than the O ring so there must be a gap.

myron1950 said:

Problem: " There is a consistent .030 to.050 gap all around cover interface."

Are the covers designed to have a gap between them with the o'ring in place?

Click to expand...

Yes, there will always be a gap and the o-ring will be visible. Considering that the attachment is a single fastener roughly in the middle of the outer cover and that the clutch side has more area than the alternator side, the compression of the o-ring will not be constant around the circumference of the cover. This is quite normal and as long as it doesn't leak you're good to go.

30 lb/ft?

Seems way too much? I have been tempted to measure the gap between the two cases on the center fixing stud and place a shim/washer on the stud such that real torque could be measured and not contribute to warping the cases, but having never had a leak torquing the center fastener with a ratchet with a 3" handle and my palm centered over the on/off switch; I've never had a reason to investigate the center gap.

I hope that you did check/set the clearance between the inner primary case and the center fixing stud, but with the uniform gap between the inner and outer cases you either did check/set the clearance or got lucky. I applaud your effort and more that you asked the question.

Best.

RoadScholar said:

30 lb/ft?

Seems way too much? I have been tempted to measure the gap between the two cases on the center fixing stud and place a shim/washer on the stud such that real torque could be measured and not contribute to warping the cases, but having never had a leak torquing the center fastener with a ratchet with a 3" handle and my palm centered over the on/off switch; I've never had a reason to investigate the center gap.

I hope that you did check/set the clearance between the inner primary case and the center fixing stud, but with the uniform gap between the inner and outer cases you either did check/set the clearance or got lucky. I applaud your effort and more that you asked the question.

Best.

Click to expand...

As the original inner case was cracked adjacent to the center fixing stud I went through a process of measure the gap between the inner case and the stud shoulder using a "trial" pack of shims, measuring the gap with feeler gages with the three crankcase bolts snuged-down with the gasket in place and then a final shim-pack that comprehended the original shim-pack and the feeler gage dimension. I am confident that I am not excessively stressing the inner case at that location.

I did clay the center stud to outer cover when I trial fitted the cover without the o-ring gasket. The clay was compressed but did not squeeze out. To your suggestion I probably should have measured what the dimension of clearance was - my bad.

The torque listed on the Old Brits torque sheet was 25 ft-lbs. The only reason for 30 ft-lbs was to see if any further torque would compress the rubber - I should have known better as rubber becomes very stiff at its compressive elastic limit.

I have reinstalled the o-ring and cover. The gap is surprisingly uniform given the single-bolt for compression - a good indication of the lateral stiffness of the cover.

I can live with the gap. I was just surprised that the joint was designed in that way. Its not too unusual to see an o-ring joint with a gap but usually it hidden by an overlapping shoulder that creates a labyrinth. Or, the joint comprehends the compressive limit of the rubber and the groove is designed for a depth that allows the two case halves to mate.

Thank you for your input - good discussion - good learning for me.