Belt or chain primary

[Bear]

I have noticed that many of you have changed to belt primary drives. There has been a variety of sizes, from 1.5 -3.0 inch belts involved. Why change, and which size is the best? (I would like to enclose it and make it close to stock look. I am trying for a little more power and torque, but not too much out of 1930's technology)
thanks,
bear

[57stroker]

I'm certainly no expert , I've only had my Pan basket running for 2 years, but I ran the chain primary for the first year and then changed to belt. I'll tell you what I learned about both.
Chain primary , The pros: You can buy different size engine sprockets to get whatever gearing you want. It's nearly bullet proof and you can find double row chain just about anywhere. Assembly and adjustment is pretty easy. The cons: The chain can cause more noise and vibration. The chain needs to be lubed from the oil pump with engine oil. Wherever you park, there will be at least a good-sized spot of oil. The oil adjusting screw will plug from time to time, then you will need to open the screw up, usually covering the back rim with spots of oil. The chain will need frequent adjustment. The drill goes something like this: you pull the inspection plate on the primary cover, loosen the tranny bolts, slid the tranny back with the adjustment screw, tighten the bolts back down, tighten the back chain (because moving the tranny back caused the back chain to loosen) and sometimes readjust the clutch and back brake.
Belt primary , The pros: Silent and maintenance free. It weighs less than the chain and you can rev the motor freely without any noticeable vibration. No mess - you can turn the oiler off. Oil leaks are easier to track down and fix, the bike stays cleaner. The Cons: Limited availability of engine pulley sizes. If you're running a rear drum brake, you're gearing options are pretty slim. Usually the bike is geared too low to run comfortably above 65 - 70 miles per hour. If the belt strips teeth or breaks, you're done. You will need to order a new belt from a parts dealer. It'll cost more than a new chain. Aligning the belt drive can be a challenge; it took me 2 days to get the belt to stay on the pulleys. Given a choice, I'd run an enclosed belt. (I like the look s of the chrome diamond primary cover.) My primary covers were too screwed up with cracks and stripped holes to save, I ended buying a 3" open belt. It works well, but I drive down a mile of graveled road everyday to get to work. I'm constantly picking rocks out of the belt and the engine pulley is starting to get scared up. After running the chain and the belt for a year each, I'd never go back to the chain. Hope all this helps, I'm sure other will have more to say. Good Luck!

[dereborn]

I run an 1,5' PRIMO belt inside diamond cover, which was mounted when i bought my ride. Has worked flawlessly for 3 seasons (about 6000 km:s per season). I am definitely going to stick with belt drive in the future as well!

BUT...

I totally agree with the gearing problem. I've put at 26 tooth on the tranny and still feel uncomfortable running highway speeds long distances. What's this 'bout the rear brake influence on this? Can you put in a bigger pulley if you change to disc? How about juice drum?
Let me in on this!

[57stroker]

If you run a rear disc and a separate sprocket, you can run any size sprocket you want. Otherwise, you are stuck with sprocket that is riveted to the brake drum.

[dereborn]

Ah... I was still stuck inside the primary... I'm actually gonna try to custom-make a smaller drum-mounted sprocket this winter. As the stock one seems to have at least 10 mm clearance between chain and drum, I'll buy a flat sprocket with, let's say 50 teeth an have the BIG hole for the drum taken out of it. Should work, or what do ya think?

[57stroker]

Try to do the math and see where that will put you RPM wise. It does look like there may be room. Most of the sprockets I've seen are welded to the drum. The rivets must give up after awhile. Mine was welded when I bought the basket case, and most of the welds were cracked. I re-welded it and then turned the drum in a lathe to true it back up again. I wonder if the welds can handle the heating/cooling cycles of the drum. If I end up welding it again, I'll probably braze it. I'm trying to go a different route. I'm going to machine a front pulley with 4 or 5 more teeth. I've done the math on the center distances of the pulleys and you end moving the tranny foward about 5/16". The owners of the machine shop where I work were going to adopt it as a "pet project" with the hopes of selling a few, but it keeps getting put on the back burner. Maybe this winter when things get slow.... With the 3" belt you're pretty much stuck with sizes offered by the belt drive companies. (Just one size as far as I know!) With 1.5" belt, there may be more sizes available, but with different splines in the center. If I can scrape up enough $$$ to change to an enclosed belt, I will see if I can get a big enough engine pulley and change the center to the Panhead size. One way or another, I'm going to make this bike cruise comfortably at 75 MPH! I'll keep you posted.

[VintageTwin]

I talked to PRIMO in the past about different size motor pulleys. He told me they had no plans of making a 24 tooth motor pulley equivalent. I went to BDL and he told me that attempts to make any motor pulley other than the 23 tooth equivalent lead to belt destruction. Whatever combination PRIMO, BDL or KARATA use is what mathmatically "works" sucessfully on the machines. The biggest bane to me is having a motor that sounds and feels like it wants another gear at 65 mph. That sucks, but I'm making my machines 1-1/2", 11 mm., under OE tin, (non- vented, stock inner and outer primary covers) belt drives anyway.

[dereborn]

Never saw a welded rear sprocket! Wonder if tradition differs in different parts of the world... Riveted of course, but also bolted. I had to change mine the other year, and was adviced to bolt it, did so and hav had no problems at all. I have standard stroke, compression and bore, SU carb and no dragracing ambitions, so I guess I don't stress it a lot. 6mm bolts with nyloc nuts anyway does the trick for me! I think it's 52 teeth now, right? how much do you guys think I have to lower that to notice any difference? 50? 48?

Cheers

[DuoDave]

A word of warning if you are dealing with a riveted rear sprocket. I had to replace mine, so I ground the heads off the rivets and went round the drum tapping them out. The last one(!) was distorted and jammed, so I gave it a slightly harder tap and an inch sqare section of the drum broke clean off. I should have remembered from past experience how brittle cast iron is. I had to buy a new drum and it was pretty nerve wracking peening the heads of the new rivets.
As dereborn says.....learn by doing. :

[VintageTwin]

Anyone know of a jig (OE or homemade) that will work for replacing rear sprockets. The '36-57 Big Twins used rivets, maybe the '58 did too, but the '59 up uses rivets and dowels pins (like a rivet but bigger or vice-versa, I don't know, I'm not looking at the book). I had a chapter going on it in 2000, but I explained and sent a '63 rear drum to USATCO (U.S. Air Tool). He sent me back a rivet air gun and bucking bar (you place this steel slug on the shank) and you put the cupped end of the rivet gun on the head of the rivet and ...rat-a-tat-tat...whilst you hold the buck bar against the shank. The amazing thing that happens is that the rivet shank sticking out from the sprocket surface squashes down into a short-stacked barrel of solid steel. Mike W. at USATCO did a few trial rivets on the '63 sprocket and drum I sent him. The barrels squashed in perfect symmetry, like a machine squashed them. I tried it on the '63 drum with my new rivet gun and backing bar next to his perfect ones and my rivet barrel squashed OK, but was kinda lopsided, but the rivet was tight and I could tell, that with practice I could do better. Problem is, that when I went to Kennedy's (in Oceanside) to find a drum in their graveyard, the only drum and sprocket I could find was this '63 that had the sprocket bolted to the drum (like Dereborn mentioned). I figured it would work and be easy to remove the sprocket from the drum for my experiment. Anyway...I did all this work, and didn't realize until I had the '63 sprocket back from USATCO, that the years I am concerned with '36-59 all have separate dust rings in front of the sprocket. '63-up has a cast dust-ring on the drum!! Awww...man!! Bum-kick!. I went back on the phone to Mike W, and he said..."Oh yeah, that does make a difference". So, if you read the Palmer resto book, he talks about needing a hollow drift. OK, but I think you need a hollow drift that has a bottom in the hollow drift's shank. The bottom in the hollow shank will act like a bucking bar. The hollow will allow you to press the dust ring tight against the drum, right? And when you hit the rivet head with the gun, it will cause the rivet shank to hit the "bottom-out" of the hollow drift and cause the rivet to squash. The drift and the hollow needs to be a somewhat bigger diameter than the rivet shank diameter, so that the shank can squash and form a barrel. I think that with the new rivets tight in the holes of the drum and sprocket (even if you have to re-drill holes because the old ones are too slopped-out), that a hand held air rivet gun and a hollow drift with a bottom in the hollow would work on a dust-ring rear drum and sprocket.

[49oldschool]

huh?

[DuoDave]

Okay Vintage Twin, I had to read it a couple of times, but I get your drift ;D now. You've made me feel positively stone age. I just bashed my new rivets with a hammer and punch.
Then again, I'm the guy who took a chunk out of his brake drum, so don't take this as a recommendation.
If I remember rightly, the dowels are bigger than the rivets.

[52 Chopper]

I'm planning to change my 52 to a open primary running a 3"and jockey shift. Does a stock kickstand work or is it too close to the belt for comfort? thanks

[kell]

I have a 3" belt and a stock kickstand, no problem.

[52 Chopper]

Yesterday with 2 good friends and many beers, we stripped down my primary to replace the chain drive with a belt and adding a jockey. What I found was what I believe was a crankcase breather stud w/nipple. What do you do with this when running an open primary?
Has anyone used the inner primary tin while running an open 3" belt and fabicated a belt gaurd for it? Any ides on making one?

Thanks