350GTR Clutch Bolts Needed - HELP!

[Stoneman]

My 350 is an early model fitted with first of three clutch design used on the model. After installing new clutch plates I was carefully putting in the clutch bolts that tension the clutch springs. Even though the bolts were not even tight, one sheared off where the threads meet the shoulder.

So now I’m trying to track down a replacement bolt and I’m kinda dead in the water until I find one. Some of these early bikes may have had this original style clutch switched out for the second model. I fear this may mean these bolts are especially hard to find. I’ll try to post a photo shortly. The threads are M6 x1.000, you can see the other dimensions in the photo. Shoulder diameter is 10mm. I’d take more than one if anyone has them to spare.

[BRT-GTR]

  As you say, you don't see many of the early clutches these days. Does yours have the loose steel thimbles that the springs sit in ? if so what is the serial number of your machine. My GTR back in 68 had the same clutch but I never made a note of it's number. I have a feeling that maybe the first 1000 or so GTRs had the early clutch.
 
  If you can't find the correct shouldered bolt, maybe you could use a standard bolt, grade 8.8 or above,  with a tubular steel spacer to simulate the shoulder, just a suggestion to get you back on the road.

[Stoneman]

Thanks for your response. My serial # is 21R00439, the 439 unit off the assembly line. Quite early, eh? (Just so happens it rolled off the line in the same month and year I graduated high school— nice coincidence) And yes, my clutch style uses the metal cups with holes in the bottom.

Yesterday I installed a make shift shoulder bolt I made exactly as you describe. Had to shorten the bolt a bit as well as the spacer. I epoxied the spacer in place on the bolt. Clutch seems to be working fine based on a short test ride. My “hack” makes me a bit nervous because if it ever came loose could be the end of an expensive new set of clutch plates.

Today I ordered off the shelf shoulder bolts that are as close as I could find to originals. Differences are shoulder diameter 2mm less, thread length 2mm less and the head is round with hex socket to tighten with a Allen wrench. I was a bit worried the head diameter is a bit small at 11 mm so ordered some m8 flat washers that will better cover the top of the spring. The bolts were about $2.50 US each and washers just pennies per piece.

Would still prefer an original bolt but have come up empty so far. 

[BRT-GTR]

       Well done with the fabricated clutch bolt. It should be fine with the spring tension constantly holding it in place. Never known one of these bolts to come loose.
       Many thanks for the serial number, that would place my original GTR being built in the first few months of production (June, July 67) which surprises me as the 350 BS did not go on sale in the UK until June 68. Maybe imports from Japan took a lot longer back then !

[Stoneman]

Somewhere on this site is an explanation of how to decode the serial numbers. Using that, determined my bike at 439 off assembly line happened in June 1967. On page 22 of the service manual are three illustrations of the each clutch modification. It indicates the first modification began with 21S01022, or the 1,022nd unit off the line.

[BRT-GTR]

   I'v given up on trying to get my head around the modifications to the GTR clutch and what the designers were trying to achieve. There are service letters indicating they were trying to quieten the clutch but they don't mention plate breakage which may have been the primary objective !
  Notice the 2nd mod also occured at machine 1022 but shows a different clutch hub to mod 1 - strange. It is also known that very early machines had a cast iron clutch hub, not alloy.
  What I would recommend is that you allow the new clutch plates a running in period without violent action. This allows the friction plate lugs to bed in and for more of them to make contact with the clutch outer drum, spreading the load when releasing the clutch. I'll wrap it up at that.

[Stoneman]

BRT-GTR, not to prolong this thread but you may be misreading the three illustrations in the service manual. The first illo is the original design and says serial numbers BEFORE 1022. The second illustration says AFTER 1022, which is the first modification of the original design. The third illustration says AFTER 4022, which would be second modification, or said another way, the 3rd design change.

Maybe you weren’t confused as I was originally, took me several readings before the light went on. And, just FYI, my clutch hub is steel, aluminum.

Lastly, thanks for the advice about a gentle break-in period for the new clutch plates.

[BRT-GTR]

    Many thanks, you were right about my having misread that 'before & after', Doh,  that makes more sense. Don't get old !!, lol.