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Old 14/01/2011, 09:21 PM
greevous greevous is offline
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Default Multi-Model Primary Case?

I was pulling off the primary chain off my '69 MX6 Challenger and noticed that there was a "seam" on the inside over the front primary sprocket, which looked like it was intended to be bored out to accomodate something on some model other than mine.

After noticing the seam on the inside I noticed the same thing on the outside (which I'd never noticed before).

And actually the whole mystery started when I pulled off the primary cover and noticed that the end of the crankshaft had a large notch in it that looked like it was able to engage with something, but I wasn't sure exactly what it was. Tach drive?

Any ideas what model would have utilized that stuff? I've attached a couple photos to perhaps illustrated a little better what I'm talking about.

-Connal

Inside of Primary ("seam" is on the right)


Outside of Primary


Notch is on the sprocket on the left
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Old 14/01/2011, 10:00 PM
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John-Boy John-Boy is offline
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The casing is identical to the one on my 36MX4, except mine has a big groove where the clutch has tried to escape.
The slot on the end of the mainshaft is intended to drive a tachometer as found on the road racing machines, the casing would need to be machined to fit the drive box.
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Old 15/01/2011, 04:43 PM
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Andy Z Andy Z is offline
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Quote:
Originally Posted by John-Boy View Post
The casing is identical to the one on my 36MX4, except mine has a big groove where the clutch has tried to escape.
The slot on the end of the mainshaft is intended to drive a tachometer as found on the road racing machines, the casing would need to be machined to fit the drive box.
Interestingly, my MX1 crank came back from Simon Bateman with the slot in the end machined off! It has been converted to needle rollers. I don't need the tacho drive slot so I'm not bothered, but a Silverstone owner might be a bit miffed if it happened to them!

Cheers,
Andrew
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Old 15/01/2011, 04:47 PM
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Rob Rob is offline
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Yes, dead right, the 'cut-out' area was removed and a separate casting fitted in on the Silverstone RES model and the Oulton. On to this fitted a Smiths rev counter gearbox. By the book, when fitting the primary chaincase outer, particularly on the earlier cases, you were supposed to remove the gearbox, use a spaecial tool to align the outer case before tightening it and then re-fit the gearbox.

Whether you did or not, the end of the crank always seemed to chew off the tang of the rev counter drive and most people didn't seem to bother.

I always found too much going on to worry about looking at the rev counter when racing and always revved the poor thing as hard as it would go! If I had a rev counter that worked, I always forgot to look at it, this usually being brought home when someone asked what I revved it to!

I had to learn to look at the thing when I moved on to Yamaha's as it was (is?) only too easy to rev to 13,000+ which is a bit naughty. Also, the water temp needed to be watched as a Yam tends to sieze a bit quicker than a good old Greeves and with little or no warning! You are also going a good bit quicker too! Happy days!

Rob
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Old 15/01/2011, 07:42 PM
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Colin Sparrow Colin Sparrow is offline
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Rob's right, the tang on the rev-counter drive chews up in no time. At least these days you can use a Scitsu or some other electronic tachometer. And you can use any Challenger chaincase on your racer.
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Old 15/01/2011, 11:14 PM
greevous greevous is offline
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Fantastic! Thanks for the info! I'll be leaving it alone, but always nice to know the what and whys of things.
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Old 21/01/2011, 10:09 PM
Stan Nicholson Stan Nicholson is offline
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My RDS has this style of primary chain case fitted, with the ¼ BSW screws around the outside. The outer case incorporates the additional section of casting for the tach drive that also has a bore on the inside housing a bearing which engages with the plain diameter on the end of the crankshaft. Assuming this is concentric with the location diameter for the tach drive , this should align the drive to reduce the wear on the tang of the drive. It looks like the pattern probably had a removable section to enable it to be used for both the moto cross and road racers, rather than welding an additional section into the case. It certainly looks like it is part of the original casting. Was this design with the bearing standard on the RDS?
I also have an RCS that is in pieces but the primary chain case on that has the 2 bolt fitting and the casting for the tach drive does not have provision for the ‘aligning’ bearing. When I used this case on my RDS, along with the 4 speed converted gearbox after my cam change 5 speed broke, I made up an aligning tool that fitted in the locating diameter of the drive and over the end of the crankshaft. Either way the tach drives do wear, although having raced the thing for 28 years now in vintage races, I guess I can’t complain.
Stan
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  #8  
Old 04/09/2017, 09:48 AM
Mike Norris Mike Norris is offline
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Default Multi-model primary chain case

Hello all.
Reviving an old thread.
I have a similar chain case on both my 250 and 360 challengers.
Over the years the primary chain has chaffed the metal round the bolt holes and recently a stone was lodged between the case and the frame during a race grinding the case very thin.
I expect that some of you will have suffered similar problems and may wish to replace the outer cover. There may be the old worn one available but I am wondering if there would be sufficient demand to have some remade.
I have spoken to Colin at Cotton Villiers Services who would be able to have them cast.
This is a good conversion for the 250 challenger as with the bolts round the outside of the case it is easy to get an oil tight seal. I do not need a gasket just a thin film of gasket cement.
Mike
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