What's up with my Z-plate / rear ISO

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Tom P

Tom P

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My 1972 750 seems to have a problem with vibration, not bad but not silky smooth. I have noticed that the left hand Z-plate and primary are very close to touching (the bottom third of the z-plate) but the top third seems to have to large a gap. Any ideas where to start looking? Is this a sign of collapsed ISO rubbers? (The PO provided receipts showing they were replaced in 2020 with a complete vernier setup from AN) or is it a symptom of incorrect assembly - missing spacers etc.
Any ideas?
Thanks
Tom

https://photos.app.goo.gl/utRALFsDDSx97peY6
 
That is a little close for comfort... Something is definitely up there. The typical collapse means that the top would have little clearance.

Can you loosen the Z-plate mounting bolts and rotate it away from the primary at all?? Shot in the dark....
 
Derek Wilson said:
That is a little close for comfort... Something is definitely up there. The typical collapse means that the top would have little clearance.

Can you loosen the Z-plate mounting bolts and rotate it away from the primary at all?? Shot in the dark....
Click to expand...
That was the first thing I tried, I managed to get about 20 thou extra clearance by rotating the Z Plate
 
Tom P said:
(The PO provided receipts showing they were replaced in 2020 with a complete vernier setup from AN) or is it a symptom of incorrect assembly - missing spacers etc.
Click to expand...

Front vernier Iso. possibly incorrectly assembled or wrong parts?
 
L.A.B. said:
Front vernier Iso. possibly incorrectly assembled or wrong parts?
Click to expand...
I was just thinking that I will have a look at the Iso's, it seems to me the entire engine is to far back, not sure however how the Iso's could cause it though
 
Tom P said:
I was just thinking that I will have a look at the Iso's, it seems to me the entire engine is to far back, not sure however how the Iso's could cause it though
Click to expand...

I was thinking that if the front end of the engine-cradle assembly was forced over to the left by the front Iso mount then that could perhaps result in a reduced gap between the primary case and Z-plate?
 
Could the frame have contacted something and been bent inward at the front?

Glen
 
worntorn said:
Could the frame have contacted something and been bent inward at the front?

Glen
Click to expand...
I hope not, the pile of receipts I got from the PO shows the frame was checked/repaired by Norman White...
'Preparation and repair of chassis prior to painting, inc. removal and repair, replacement of rear loop. Braze and dress tube imperfections, repair damaged lower rail and replace side stand lug and Gusset swingarm'

I would like to imagine that he would have checked the alignment and critical mounting points
 
Tom P said:
Hmm, just had a quick look at the front Iso (as installed) there is a difference in the distances between the engine mounting lugs on the frame and the 'ears' of the iso mount

https://photos.app.goo.gl/vQWvNKBSZAtT4yPq8
Click to expand...
There is a difference between a Mark III front ISO mount and the vernier conversion for the Mark II and earlier. The Mark III ISO is offset to allow for the thicker adjuster well the Mark II vernier conversion use the thin adjuster. What you have there looks like the Mark III front ISO.
 
Dan1950 said:
There is a difference between a Mark III front ISO mount and the vernier conversion for the Mark II and earlier. The Mark III ISO is offset to allow for the thicker adjuster well the Mark II vernier conversion use the thin adjuster. What you have there looks like the Mark III front ISO.
Click to expand...
I think you are right, looking thru the box of receipts, I see 'Iso conversion kit 06.7116, would this cause the engine to be canted? If so I looks like I need to replace both the front engine mounting (06.1410), and the vernier iso (06.7337)
 
Tom P said:
Hmm, just had a quick look at the front Iso (as installed) there is a difference in the distances between the engine mounting lugs on the frame and the 'ears' of the iso mount

https://photos.app.goo.gl/vQWvNKBSZAtT4yPq8
Click to expand...

A difference is to be expected as the cradle is offset to the left of the frame centreline.

Tom P said:
I think you are right, looking thru the box of receipts, I see 'Iso conversion kit 06.7116, would this cause the engine to be canted? If so I looks like I need to replace both the front engine mounting (06.1410), and the vernier iso (06.7337)
Click to expand...

06.7116 is the Mk3 only kit, not the pre-Mk3 conversion kit 06.7337 so it depends whether the shorter Mk3 front Iso. mounting has also been fitted or the original Iso. tube has been machined down to the Mk3 tube length?
 
Tom P said:
Hmm, just had a quick look at the front Iso (as installed) there is a difference in the distances between the engine mounting lugs on the frame and the 'ears' of the iso mount

https://photos.app.goo.gl/vQWvNKBSZAtT4yPq8
Click to expand...
This difference in dimensions is normal. The engine is offset to the left in a Commando.

And fitting a Mk3 front isolastic in place of a Mk2 retrofit kit is not unusual either, it just means that the front housing has to be machined narrower on the one side (like a Mk3 is from the factory).
 
Tom P said:
I hope not, the pile of receipts I got from the PO shows the frame was checked/repaired by Norman White...
< .... >
I would like to imagine that he would have checked the alignment and critical mounting points
Click to expand...
Why don't you give Norman a call? He should know if a jig was used for straigtening and repair. There are checking dimensions in the Workshop manual .... A professional frame repair should have a measurement report attached.
I hope for your sake all critical dimensions are within tolerance.

If the bike has been run for some time with loose engine mounts, it's possible the bolt holes in frame lugs and cradle have deformed. The final drive will try to pull the powerplant backwards, which may explain a too small gap.

Have you established size of the gap on an "as new" bike?

- Knut
 
Last edited:
Hmm, seeing that the Iso kit does fit into the housing I'm assuming that the front housing is ok. So assuming that the housing is a mk3 (or machined) the mk3 iso kit is not the problem
 
Derek Wilson said:
This difference in dimensions is normal. The engine is offset to the left in a Commando.

And fitting a Mk3 front isolastic in place of a Mk2 retrofit kit is not unusual either, it just means that the front housing has to be machined narrower on the one side (like a Mk3 is from the factory).
Click to expand...
My '74 MK II has a MK III front iso.
 
The principle visible difference is the Mk3 adjuster nut has a large chamfer on the out board side, the Mk2 adapter does not have this, as the adjuster nut is narrower.

Personally, I like the Mk3 one better, as there is more room to get in there to adjust it, although it is not a direct fit in a non-Mk3 front mount.

@Tom P - yours looks like a Mk3
 
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