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Here's a slightly odd one..... over the winter I've been thoroughly checking over the 1975 350 Mark3 I bought last summer, there are a couple of other posts I put on here about the same bike.
Washing out the old crud from the forks I found the two stanchion clamping bolts in the top yolk are stripped at the same point in the thread, but the bolts in the bottom yolk are fine (one example of each in the photograph).
The yolks have been powder coated and the stanchions are a very tight fit. It seems unlikely, but is it possible that the top bolts should be shorter, rather than (my suspicion) that (the top) yolk threads weren't plugged or cleaned out properly. I don't want to get the tap and die set out if the bolts are actually the wrong length.
Second question. The rear (standard) shocks are pretty underdamped and bouncy. Are they like that as standard? Should I strip and service the shocks or buy a set of Hagons?
Thanks in anticipation.

Poke a thin screwdriver or something into the hole and measure the depth compared to the bolt length.

Hiya9612 wrote: <snip> ... is it possible that the top bolts should be shorter

According to the 450 Mark 3 1974 parts book I have, the top clamp bolts are part no. 71502.2615 and are M10x50.
Unfortunately, the bottom bolts are neither illustrated nor listed! Grrrrr .....

This is for the Ceriani fork, btw.
Edit: WRONG. Marzocchi. Thanks George and Jon. My apologies.

To confuse matters further, both sets of bolts, top and bottom for the Desmo are different: part no. 71502.2609, M10x40. But this is for the Ceriani fork as fitted to the 1974 disc brake Desmo.

Me, without more knowledgeable info from others, I would probably fit M10x40s, top and bottom.

Hiya9612 wrote:The rear (standard) shocks are pretty underdamped and bouncy. Are they like that as standard? Should I strip and service the shocks or buy a set of Hagons?

They will of course be totally flogged by now. And they weren't great new. Rebuild them if you like originality, replace with Hagons or whatever you like if you want something that works . A cheap Chinese set will be better than what you have now.

If the yokes have been powder coated and the threads not plugged you will have to get the taps out and clean them out, once you run a tap down the hole you will then know exactly what length of bolt you need. I assume your forks are Marzocchi? thats what a 75 mk3 350 came with I have the same bike and I am pretty sure top and bottom yokes are the same casting and the same bolts?. The rear shocks will be shot! if you are not mad into originality just replace them with pref. Hagons, you can repair the Marzocchi's but it will be expensive and a bit tricky.
Cheers,
George

The top and bottom pinch bolts on the late Mark 3 Marzocchi yokes are different depending on whether you have standard handlebars or clip-ons.
The standard bars with clamps on the top yoke use M10 bolts, but the smooth top yoke with the clip-ons use M8 bolts.
The bottom Marzocchi yoke use the same M10 bolts as the top one with the standard bars.

I agree with Craig re. the old Marzocchi rear dampers. They were never much use new. They can be rebuilt though.
I use Hagons but find the rear springs too stiff as supplied.
The standard Hagon shocks as supplied have 14kg/cm deflection fitted. I always get them to fit the 12kg/cm springs.

Jon

Thanks for all the contributions but...... this evening I found the cause of my woes and (I shouldn't be surprised by this but.....) it turns out that mounting holes for the instrument cluster actually intersect with the drillings for the fork clamp bolts and the person of dubious parentage who laughably "restored" my bike either fitted overlong instru' cluster bolts or (maybe) left out a couple of spacers. Either way, the vertical bolts stripped out the clamp bolts.

Ah ... I had wondered just HOW the damage had occurred.

Good. Sorted