Tim O'Mahony's 1978 900SD Darmah

tim61 · Post by **tim61** » Sun Feb 26, 2006 9:28 pm

So, off to Phil in Oz for a back-cut...

...and no more 1st gear ProStock launches!!!

tim61 · Post by **tim61** » Tue Feb 28, 2006 2:27 pm

Here you can clearly see the damage to shift fork caused by first gear popping out and pushing back against the fork.

tim61 · Post by **tim61** » Tue Feb 28, 2006 2:29 pm

This is a shot of the damage to the second gear dogs. The rest of the transmission looks pretty good.

Geoff · Post by **Geoff** » Thu Mar 02, 2006 9:30 am

Tim, excuse my ignorance, but what is 'back-cutting' and why do you have to do it? And what are the risks or other mods you'd need to make after back-cutting?
Cheers
Geoff

tim61 · Post by **tim61** » Thu Mar 02, 2006 9:19 pm

"Back Cut refers to cutting the engagement dogs at an angle (in the case of this photo' 3 degrees) so that once they begin to engage, they stay that way. If they don't stay engaged, the end result is rounded dogs, like the photos above.

Geoff · Post by **Geoff** » Fri Mar 03, 2006 7:34 am

Thanks Tim; sounds like a worthwhile mod if the 'box is apart anyway.
Cheers!

tim61 · Post by **tim61** » Fri Jul 07, 2006 9:50 am

After 4 months of down time waiting for the heads and crank, I am finally reassembling the Darmah engine. The work that Guy Martin at Martin Brickwood Performance did on the heads and crank looks outstanding. On the heads, he installed hardened seats, new valves, new full length guides with standard seals, machined the closing rockers to clear the guides, Modified the rockers/rocker arms to allow valve adjustments without removing rocker shafts (ala Paso/F1), port matched the intakes and installed a set of MBP collets.

tim61 · Post by **tim61** » Fri Jul 07, 2006 9:53 am

I also used a set of black anodized aluminum intake spacers from Dr. Desmo.

tim61 · Post by **tim61** » Fri Jul 07, 2006 9:54 am

Guy bead blasted the heads, adjusted the valves and set all the upper bevel shims in the process. Expensive??? YES! But worth it to me!

tim61 · Post by **tim61** » Fri Jul 07, 2006 9:59 am

The MBP "bulletproof" crank is pretty, but the fine work is not as apparent to the eye. Guy has the rods machined to accept a Stainless insert on the big end, and then uses a heavy duty bearing and a rod pin of his own design. I have heard of several vintage racers with more than 8 seasons on these cranks with no failures! Again, pricey, but a bargain compared to a crank failure!

Geoff · Post by **Geoff** » Fri Jul 07, 2006 10:00 am

Saw your comments on the Heads list too; they look great Tim. I certainly will go that route should I need work on mine! I'm jealous of your dedication and willingness to tackle the engine rebuild yourself; getting access to the tools was helpful I'm sure - and the occasional look over the shoulders from the other Tim a blessing!
Cheers
Geoff

tim61 · Post by **tim61** » Fri Jul 07, 2006 10:27 am

Geoff,

Thanks for the kind words of encouragement! I think I figured out the picture posting thing again. I post about once every 6 months, which is just about how long it takes me to forget how to do it!

As far as the rebuild, it was a lot easier than I thought it would be. It just takes a lot of time. I was actually quite entertained by the shimming process. Having only worked on Japanese bikes up to this point, I was well versed in the "assemble it and go" theory. The bevel requires a much more involved process which is really a lot of fun, once you get going

It becomes a game of seeing just how close to perfect you can get the clearance on all the shafts. Here are some pictures of that process.

tim61 · Post by **tim61** » Fri Jul 07, 2006 10:32 am

Layshaft Shims

tim61 · Post by **tim61** » Fri Jul 07, 2006 10:34 am

Then, after double checking, triple checking, and quadruple checking everything, I am ready to put the case halves together

tim61 · Post by **tim61** » Fri Jul 07, 2006 10:37 am

And, Presto! The completed, hopefully long lasting bottom end assembly!

<-=- Club Bevel Heaven -=->

Tim O'Mahony's 1978 900SD Darmah

Going back together!