Front isolastic fitment uncertainty

BERT

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This 72 came with the conversion kit installed. I'm not sure if there are shims involved setting up this kit. There were no shims when I took it apart, only the .062" grey plastic washers on each side between the engine mounts and iso caps. The engine mount end caps were both missing. The dimension of the pre-assembly is 5.960" including the mount, 2 end caps, 2 plastic washers, 2 iso ends and the .010" clearance. The frame tabs are 6.000" apart. Should there be shims somewhere to make up the .040"? Thanks in advance guys for help getting this sorted out.
20210418_134012.jpg
 
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This 72 came with the conversion kit installed. I'm not sure if there are shims involved setting up this kit. There were no shims when I took it apart, only the .062" grey plastic washers on each side between the engine mounts and iso caps. The engine mount end caps were both missing. The dimension of the pre-assembly is 5.960" including the mount, 2 end caps, 2 plastic washers, 2 iso ends and the .010" clearance. The frame tabs are 6.000" apart. Should there be shims somewhere to make up the .040"? Thanks in advance guys for help getting this sorted out. View attachment 79302
no shims, those are adjustable isolastics. There are many different threads detailing how to set them, but you don't need more shims.
 

L.A.B.

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This 72 came with the conversion kit installed. I'm not sure if there are shims involved setting up this kit. There were no shims when I took it apart,

There are no shims in the kit and as a rule, none should be required.
e99db066efcfe2c6c2a018e8862ada9d.jpg




The dimension of the pre-assembly is 5.960" including the mount, 2 end caps, 2 plastic washers, 2 iso ends and the .010" clearance. The frame tabs are 6.000" apart. Should there be shims somewhere to make up the .040"?

It's possible either the kit or the mount tube is a little undersized (as the usual complaint seems to have been that the assembly is too tight!
However, when the through bolt is tightened it will pull the frame tabs up to the Iso. caps but you could add shims if you like.
 

BERT

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There are no shims in the kit and as a rule, none should be required.
e99db066efcfe2c6c2a018e8862ada9d.jpg






It's possible either the kit or the mount tube is a little undersized (as the usual complaint seems to have been that the assembly is too tight!
However, when the through bolt is tightened it will pull the frame tabs up to the Iso. caps but you could add shims if you like.
The frame dimension in the workshop manual shows such a close tolerance I was questioning the sense of reefing on the bolt to squish the tabs together that much. Until you suggested the mount tube may be undersized, I had forgotten I had the ends squared up! So I'll shim accordingly. Thanks for your help sorting this out.
 

Carl H

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The frame dimension in the workshop manual shows such a close tolerance I was questioning the sense of reefing on the bolt to squish the tabs together that much. Until you suggested the mount tube may be undersized, I had forgotten I had the ends squared up! So I'll shim accordingly. Thanks for your help sorting this out.
The place to put the shim is between the frame tab and the iso end cap[ or abutment , You would need to find or make a thin washer approx .040" with a 1/2" ID. That would be the simplest way. If the whole engine /trans unit was in the bike and the rear iso Through stud nuts were torqued up . You might see the gap in the front on one side or the other (on the left or right side) and put the shim there. That would be ideal in my book.
 

grandpaul

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I've found more than one frame that had been spread apart more than .040". I just used a bit of allthread rod, some large washers, and a couple of nuts to slowly tweak them back together, then let them spring back; measure and repeat a bit at time till they were good.

Also had one or two that were squeezed together too tightly. I just reversed the process with the nuts on the inside faces of the tabs.
 
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not sure how relevant this is to the thread, but i VAGUELY recall that when retrofitting a Mk3 iso mounts to my 74-Mk2 bike, there were two AN kits available. one was plug an play using the 74's factory front mount. the other was the a straight 75, Mk3 kit but required modifying the front mount or using the Mk3 mount. i bought the AN, plug and play kit from old britts, and don't remember any additional shims
 

BERT

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as a follow up to the Mk3 iso conversion on a pre-Mk3 bike -- https://www.oldbritts.com/13_067116.html . not sure what's currently available directly from AN.

as a follow up to the Mk3 iso conversion on a pre-Mk3 bike -- https://www.oldbritts.com/13_067116.html . not sure what's currently available directly from AN.
Thank you for the links. The front engine mount is squared off on one end. I will shim the difference on one side or the other as was previously suggested when the power unit goes back in the frame for the final touch up alignment. I appreciate the input from you fellows.
 

BERT

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The place to put the shim is between the frame tab and the iso end cap[ or abutment , You would need to find or make a thin washer approx .040" with a 1/2" ID. That would be the simplest way. If the whole engine /trans unit was in the bike and the rear iso Through stud nuts were torqued up . You might see the gap in the front on one side or the other (on the left or right side) and put the shim there. That would be ideal in my book.
Makes sense to me. Thanks.
 
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I think if you measured a few front Iso's I'm sure you would find a variation in sizes. Those plastic washers wear and you take up the wear by adjusting the threaded end piece. This changes the overall length of the ISO. If you want it to be a constant length you will need to make a new shim each time you adjust the ISO which is at best inconvenient.

Personally I would just fit it without any shim and not worry.
 

Carl H

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I think if you measured a few front Iso's I'm sure you would find a variation in sizes. Those plastic washers wear and you take up the wear by adjusting the threaded end piece. This changes the overall length of the ISO. If you want it to be a constant length you will need to make a new shim each time you adjust the ISO which is at best inconvenient.

Personally I would just fit it without any shim and not worry.
The late new and improved type PTFE thrust washers are impregnated bronze and do not wear much, They are usually brownish in color. Plastic washers?? Sound like cheap imitations. Look for real OEM washers 06-4748. It would be very hard to replace them in the MK3 type isolastics once installed so why take a chance??

Some early bikes ( around 1971) OEM came with thrust washers ( they were red in color) that would break apart like "potato chips" being too brittle and then the handling went to crap and the complains began because of all the play in the suspension at the swing arm.

The reason for the frame tab width shim is to not pinch the frame. And the slight less width of the iso mount is OK because it helps you install the mount in the frame easier and then add the shim. out board to take up the 40 thou space. with a shim.
 

Carl H

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Makes sense to me. Thanks.
I'm a very fussy mechanic and I align my wheels , so part of the alignment starts with the iso mounts as it effects the location on the swing arm and rear wheel.
 

BERT

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I'm a very fussy mechanic and I align my wheels , so part of the alignment starts with the iso mounts as it effects the location on the swing arm and rear wheel.
I'm at the bare frame right now, and after addressing several questionable issues I inherited with this 72 this is the time to get the machine all good. I have removed all the powdercoating from the iso tabs, cradle mountings (swinging arm, engine, kickstand),etc. The frame and swinging arm have been inspected for true so I believe these issues were from previous assembly mistakes and will be sorted out with careful reassembly. I understand the importance of the iso mounts with alignment and really appreciate your input Carl. Thanks.
 

BERT

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The OEM ones cost approx 8- 10 $ each. So is it a cost thing??? . I know they came in MK3s and they are Great. I found them on Norvil under 063556
No, it isn't a cost thing. What does OEM mean?
 

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Original Equipment Manufacturer (NVT) and unless NOS unlikely.

I did notice the RGM (Tan) discs has a slicker side so I put that against the end caps.
 
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