View Full Version : 640 rear drum spokes short - is it just me?

05-31-2005, 04:36 AM
they are short ay factory 8.5" L. Can't achive correct off-set for proper sprocket alignment with tire centered to frame. I don't care if I have to cut down or add onto spacers.

My previous owner - (40 plus years ago) was running this bike with the sprockets misaligned approx .75". Happens to be the additional amount of off-set that I require. He had the wheel cocked to the left to try to align. Brake arm is bent to allow this. Original front sprocket heavily worn on inside only. The rear was shot most likely due to stretched out chain.

Not wanting to repeat this mess I'm real curious if anything was ever printed in the service notes to dealers? Did they just let this one go? As it was the first year of the full skirted bikes.

Or is it just me that has the lucky 640?

Off-set stock was 1.5". I need 2.10" minimum -to make this work.

Now I have to calculate the additional length of spoke that I need for Buchanans - So they won't consider me just another raveing lunatic.
I'll check the wheel to wheel before I do.

HAS ANYONE ELSE HAD THIS PROBLEM? Please ask owners for me if you can.
Or on the sly - look at axel adjustment bolt length each side. Or from ground look down the line of chain bottom to front sprocket. If it hooks to the left in relation to the face line of rear sprocket - that would indicate a problem.

06-02-2005, 02:48 AM
I now have six confirmed simular 640 cases. So I'm not crazy.

Looks like my off-set needs to be 2.5". Giving me clearance of 3/8" from tire to chain. Not ideal as that's really tight. But it's corrected now.

Priorities: power transfer, wheel alignment, brake plate/arm alignment, fender alignment.

With the spacers removed and ignoring the sub-frame. The wheel can be slid from side to side along the axel. A straight edge pinned across outer face of rear sprocket should point and rest beside outer face of front sprocket. Once these are in alignment you can measure the amount of off-set required to center the wheel to the center line of the main frame. The sub frame can be bent in or out to accomodate the spacers and/or skirted fender.

Do you have a 640? What's your rear wheel off-set? Are your sprockets aligned?

Crummy photo shows rim with 1.75" off-set. An additional 0.75" needed to center the tire.

Working with Kenny Buchanan to try to figure this out. There a good company and they do care. But as we all know- old iron isn't all standard. It's needs some TLC on assembly.

Thankfully I'm not a heavy drinker. But I could be after this 640 resto. It's been a real trip figuring out what they did in the Springfield factory. Chiefs are really well designed. Scouts are ahhhh.... individually unique.

Once the rear wheel is aligned. Do a wheel to wheel. I don't have a straight edge (2x4, L-bracket, florecent tube) handy so some dental floss used as a plumb line is fairly accurate. Taped approx. 6" up from ground to leading edge of front wheel right side - and strung to rear. You can adjust the alignment with rear axel adjustment bolts. With wheels in alignment with each other count the number of exposed threads on axel adjustment bolts. Write it down in your rider hand book and stick it in your tool box. Now when the chain is put on the bike you can do the math to calculate and retain the alignment under proper chain tension. I personally punch a small mark into one face of the axel adjustment bolt as reference. These bolts should have lock nuts.

Hope this helps a few folks. Do you have a different method?

06-02-2005, 10:22 PM
Hey AdminGuy! Although it's looking like your running into speed bumps, the photo that you supplied looks like your almost there! I hope to see the beast on the road this summer! I'm also sitting here wondering what gremlins I'm going to run into with mine. Do you know if the rear suspension models suffer from similar problems?

06-03-2005, 03:57 AM
Hey Cory, Massive head ache brewing, and also pretty heart breaking getting this far along with the project and getting broad sided.

Have added a few period correct things to style up the bobber. Keeps my spirits up as project lumbers along. Your going to love it.

But it has to work correctly if I'm accually going to ride it. Yes, I'm really pushing to get it running asap. Need run-in miles.

I'm thinking that a 1/4" to the left of center for both wheels may be a much better set-up. A little more room for the chain to float around. My new chain has approx. 0.5" deflection from side to side between sprockets.

The rear off-set will be set up a little radical simular to a Norton.

I'm not entirely sure what you may run into. I can see that they added 0.75" to the shaft and drive gear for the 41-42 models. Which would give you wheel at frame center with 1.5" off-set. You should be fine. Just pull off the rear fender and check it as outlined above. What is the total width of your 16" tire?

A 41 shaft is a no go in my case as the kicker will not align.

Take a look at pg 53-6 op & mait manu 741-640. Bottom of page shows wheels being aligned to each other. Halarious because it's a mute point if your transmission and sprockets are not in the equation.

Kiwi cato pg 58 second column a/37-45. A 640 would certainly fall into the "oddball year variation" catagory.

It's just the 640s - when they went skirted. Fred said it quite nicely as, "style driven design". I'm shooting for form and function.

On the up side! I'll bet putting together a 36 knuck is no walk in the park.

06-07-2005, 07:35 PM
Oooohhh ya. The 34-39 rear hubs were 0.75" inch shorter. So it's easy to see where the whole thing went sideways in 1940 when they introduced the skirted bikes and needed clearance for the fender. But sadly didn't extend the front sprocket (main shaft & drive gear) until the 41 models. Not like this doesn't still happen today in manufacturing or anything. Oh well..... can be corrected.

I've reverse engineered the rear and have new spokes coming. The most difficult thing I found was trying to approximate the amount of deflection that the chain sag may have under proper tension and running. I think I'll be safe.

I'll post all measurements here after I've road tested the bike and am certain that everything works perfectly.

06-07-2005, 09:29 PM
Right on AdminGuy! It'll be nice to learn something from your hardworking studies! Any info will be treasured!

06-08-2005, 07:18 PM
Maybe I missed a thead, but the transmission cases on the 640 and skirt scouts are wider than the preskirt. A skirt output shaft will fit a preskirt transmission. That will make the kicker miss. I can't remember the difference between the two transmission cases, but they are very obvious, it is like webs on the output side or something. You might have the wrong case. If that is the case, don't worry, you will find one. It is likely, if that is the case, that somebody grafted a 741 (AKA preskirt trans) into it. Could be worse. Good luck, I've gotten into worse messes and gotten out; there are far more people looking for the narrower cases than the opposite.

06-08-2005, 11:22 PM
Good info to know inrust! Could that be your trouble AdminGuy?

06-09-2005, 02:40 AM
I appreciate your interest and help. I wish it were that simple. I've got the stock 640 trans case. One support web (buttress) on left of output side. It's all factory correct stuff.

This is a known issue that I've confirmed with other 640 owners.

06-09-2005, 03:06 PM
Ziggy was kind enough to email me photos of all hubs and comparisions yr by yr. Thought this info could help someone else maybe. Sadly, I have the correct 640 hub. I guess you could simply install a 34-39 rear hub to correct the sprocket alignment problem. A 640 hub has a welded reinforced two piece backing plate on the drum. A perimeter arc weld with two horizontal welds joining the two C-shaped plates together. Measurements taken from drum flange to opposite end. Sprocket off.

741= 7-3/8" wide, j slot spoke holes
34-39= 6-5/8 wide, no welded reinforcement
640= 6-5/8 wide, welded reinforcement
41/42 = 7-3/8 wide, *welded reinforcement & no welded reinforcement used

*earily 641 had the central hub tube welded to the drum


06-11-2005, 04:30 AM
Measured from drum flange to outer face of sprocket.

Pre-skirt SS 34-39: 5 5/8"
640 SS: 6 3/8"

- exactly a 3/4" difference. Figure that one out.....

06-13-2005, 08:39 PM
i need to dissagree with your last post
the 34-39 ss hub is EXACTLY the same as the 40 ss hub with the exception of the welded backing plate
there is no 3/4 " difference nor any difference in any measurements .
i just did a full comparason between several
by chance do you have a modified hub? id love to see why you have a difference

06-15-2005, 04:00 AM
Hey Zig, I appreciate your help.
My hub is 6 5/8" height. Does not appear to be altered. Was spoked when I got it.

OK - forget about the 3/4" diff.

Do you have a 640 sub-frame, trans box w/ drive gear and shaft, rear hub. Stick them together. no spacers. You don't even need a hollow axel or anything. Just a 1/2" rod. Drive gear sprocket collar is 0.6". I have a 1/4" gap between front sprocket and trans box. Rear sprocket is flat and measures just under 3/8" thick.

Align the sprockets. And measure the off-set. Does it appear that it will even come close to the middle of the main frame opening top? Opening approx. 3". with 1.5" off-set. 2.10" to rim center. If yours does. Take a couple pictures with a t-square or something straight on center of tire/rim, and between sprockets. and email it to me. I'll buy you a case if it's bang on the money.

Is that possible? Could you do that? If it's too much trouble - I'll understand.
Can I give you any other measurements to help?

08-10-2005, 12:33 PM
My rear off-set is 2 1/8". The chain is close to the Coker firestone tire. It doesn't contact/rub in motion. The sprockets are not perfectly aligned. But they are much, much closer. Approx 3/8" out. As opposed to 3/4" out.

The best way to determine the offset required for an old motorcycle is to reverse engineer the thing. Nothing is carved in stone and each bike is most likely unique.