Hole patterns for belt drive primary

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Hortons Norton

Oct 12, 2007
Country flag
Just getting ready to drill the holes in the primary and was wondering if you guys might have some photos and advice as to how you did it. I have made up a jig that fits in the adjustment hole to mark the distance and now am wondering the best way to mark the placement as far as degrees go. It looks as if 8, 1 1/2" holes would work well but I would like other thoughts on other ways or patterns to use, Thanks guys. I really want to do this right the first time, I would love some photos, Chuck. :D :?:
On mine I machined 3 additional holes at the front part of the case, the same size as the timing inspection hole, giving four holes symetrically placed, I then machined another 5 holes in the rear the same size as the clutch adjusting cover hole, concentrically around this adjusting hole. The orientation of these holes can vary a little bit without affecting the look since being an uneven number they do not have to sit in any particular plain. I used an old cover that had been badly damaged by a foot-peg going through it, the damaged area dictated the hole position, I thus recycled something that might otherwise be thrown away. I can't post a picture from my computer, although L.A.B. kindly posted a picture of my bike on the 'Tall tales...What ever made me buy a Norton' thread, on Page 5 of that thread.
case venting

Why bother drilling a perfectly good primary cover ? When I coverted to belt drive 3 summers ago I followed Dave Comeau's advice and simply removed the tin plates from the inner cover. He had reported running his bike like that for 10 years with no problem, and I have yet to have any problem with mine. I did however, drill a small (tap-able therefore repairable) hole in the lowest spot in the cover just in case water finds its way in.
Well to be honest its about looks, I have a couple and wanted to try doing one. Your totally right It may be ruined but these things are easy to find and when done well I think they look great. Chuck. :cry:
An alternative, just replace the inspection caps with aluminum mesh (from my son's hot hatch Citroen Saxo period). Held in with a tight fit and clear RTV. Easily pulled and replaced when needed.
Hole patterns for belt drive primary
Keith: I have done the same thing with many bikes and like the idea of not drilling holes in a great looking primary cover. Good ones are getting pretty scarce on e-bay and pricy!
A benefit that noticed on a Ducati clutch cover was that any object that spins in a circular pattern generates a suction at the center. It pulls fresh air into the middle of the clutch and pressurizes the outer cover. Any opening after that will allow the warmer air to exit the now pressurized primary.
I have inspected belt drives that failed due to the large amount of rocks, dirt,etc that were ingested by the drilling of big holes in the primary covers.

If large holes are needed than a routine inspect/clean should be done.

Keith1069 said:
An alternative, just replace the inspection caps with aluminum mesh (from my son's hot hatch Citroen Saxo period). Held in with a tight fit and clear RTV. Easily pulled and replaced when needed.
Hole patterns for belt drive primary

That is one cool Commando. 8) 8)
Well thanks for your thoughts guys, I still think it does no harm and will also add to the cooling of the belt and clutch, As far as rocks and dirt go I don't plan on doing much off roading on this bike. My 98 S1 Buell has a belt final drive instead of a chain and it is exposed to the elements with no cover and the bike has over 25,000 miles on it and it looks as if it could go another 25,000 and it has 90 H.P. and I ride that bike hard. I really didn't want this to turn into a thread on if it is needed or not just some pictures from other people that have done it and how they laid it out. But I do thank you for your thoughts and once again you are right in your thinking so I do not wish to start a thread that gets off subject and turns into an argument. Thanks again and have a great day and ride safe, Chuck. :D :D :D
Thanks Mike, the result of many years of 'improvement', more comfortable to ride at speed now but the bars kill my wrists after a couple of hours. Didn't you have the 880 that exploded? Did you ever get it fixed or has it gone?
I didn't drill any holes in the cover.
I made a cut-out in the inner backplate inbetweem the engine and gearbox, close to the lower part of the cover. This I then covered with aluminium mesh. So you get the cooling without the visual difference.
It also protects against splash ans splatter.
It seems to work fine.
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