Dynamo Strap 67-1876 - How Tight?

[Petergj]

Folks, does anyone have a recommendation for how tight the nut on the dynamo securing strap should be? I have recently converted my A10 to belt drive and fitted some high temperature rubber strip between the dynamo and the strap to help prevent dynamo body rotation (and hence a slack belt). Unfortunately the dynamo has slipped in the strap. I'm hesitant to overtighten the nut in case I distort the body of the dynamo as reported by another contributor on this excellent forum.
Any comments much appreciated.

[Rex]

I can't think you'd distort the heavy dynamo body using the existing design of strap, unless you somehow went really silly on it. The belt drive shouldn't exert that much strain on the dynamo or alter it's position, and shouldn't be anywhere near bar-tight anyway.
Someone will say that those rubber strips will now insulate the dynamo yoke electrically, too..

[berger]

you might find you have dynamo creep when tightening the strap , i did and had to compensate for it, as for the rubber whatever it is even a bit of paint i put on years ago when i was younger and a little bit dafter stopped mine working.

[Joolstacho]

Peter the dynamo bodies are REALLY strong, I don't see how the strap could ever distort it. So tighten up the strap pretty hard.

[JulianS]

I would remove the rubber strip and do up the nut really tight.
I agree the bodies are very robust and unlikely to distort.
The worst that could happen is that the strap pivot pin in the crankcase bends.
I have never had a dynamo body distort in 52 years of use of my A10.

[trevinoz]

Don't go overboard when tightening.
I have seen a crankcase broken out on the pin.

[muskrat]

Don't go overboard when tightening.
I have seen a crankcase broken out on the pin.

DAMHIK. One of the reasons I replaced the cases on last rebuild 
Cheers.

[CheeserBeezer]

I can't think you'd distort the heavy dynamo body using the existing design of strap, unless you somehow went really silly on it. The belt drive shouldn't exert that much strain on the dynamo or alter it's position, and shouldn't be anywhere near bar-tight anyway.
Someone will say that those rubber strips will now insulate the dynamo yoke electrically, too..

Unfortunately, distortion of the body does happen, not by much, but enough to cause interference between the armature and the body or pole shoe. The problem is that the quality of engineering of the dynamos is quite poor. The cases aren't always round to start with! The armatures run very close to the body and the pole shoe and, in some case, it might only take the distortion of a thou or two to cause interference. Thankfully, you shouldn't have to tighten the strap too much. Most importantly, make sure the belt (or chain for those who use them) isn't running too tight as that will cause the dynamo to slip. Make sure you don't have any tight spots. I have come across dynamos which run fine on the bench, but as soon as you fit them to the bike the armature catches the pole shoe.

[Colsbeeza]

I have stuck with the chain. No tension if allowed a bit of slack, so don't need to tighten the strap much at all. Col

[chaterlea25]

Hi All,
I use a small G clamp to hold the dynamo from moving when tightening the strap
There is a flat section on the eccentric plate that sits against the casing that the clamp can grip.. it also ensures that the cork washer is tight against the case.
I agree with the other comments, dump the rubber.
I use a short ring spanner and tighten until it feels solid

John

[jjbsa]

When putting a dynamo on, I fit it loose and then rotate it to get the chain or belt tension as I want it, then I put a spot with a fibre tip pen across the gap between the Mazak holder for the cork washer and the body of the engine, see photo.
Then when I tighten the nut on the strap bolt I can see if the dynamo is rotating, it usually does!  If it does rotate, then pause when the bolt is just starting to turn the dynamo and, using a long large screwdriver as a punch, apply the screwdriver tip to the shoulder on the holder for the cork washer (lower RHS in photo) and gently tap the handle end.  The dynamo will rotate anticlockwise. You may need to rotate the dynamo past the point where the fibe tip marks are in line, but it works.  Do it iteratively, though you may need to slacken off the nut once you have it fairly well tightened.  Of course, check the belt or chain tension as you go.  I don't tighten the bolt too much as I've seen engines with bent strap retaining studs. Just a medium pull on a little ring spanner about 3 1/2 inches long is enough and I've never had a dynamo move (I leave the fibre tip dot on permanently).  Make sure there is no grit etc. beween the crankcase and the dynamo when starting too!

I hope that helps,

Jon

[groily]

. . . I don't tighten the bolt too much as I've seen engines with bent strap retaining studs. Just a medium pull on a little ring spanner about 3 1/2 inches long is enough  . . .
Jon

How true is that. Or shear, even, which is made worse if you can't get onto the remains because they're below 'alloy' level.
Took me a full day to sort a busted stud on one of mine as the engine had to come out for me to get weapons onto the remnants. Made worse for not being a blind hole, so enthusiastic drilling could have put swarf inside and required an engine strip.

[berger]

jjbsa i have used your method in past years , good innit