Stripped earth lead bolt at gearbox

[kirby morgan]

Guys,
Hoping to help a mate out with his R100RS (pre 1980).
The earth lead bolt at the gearbox casing has been overtightened in the past and caused a stripped thread to the engine casing (probably caused by a previous owner putting in a stainless steel bolt) and consequently he has had the occasional starting problem.
He has noticed this earthing bolt has a hole/vent through it.
Question is, is the vented bolt crucial, or can we replace with an ordinary bolt, and what's the best method to rectify this stripped thread.
I've suggested relocating the earth lead to another gearbox bolt, but I think he wants to keep it original.
Access to this bolt area is restricted by the frame, so any tips please.

 

[cookie]

relocating the earth is the simplest option. you do need the hole in the bolt, it's a vent for the gearbox.

i have successfully put a helicoil in that hole without disturbing the swing arm etc. wasn't easy though

IIRC i used a long 1/4" drive extension with a tap on the end. it would be much easier to do if you took the swing arm out. oh, and you need to shorten the helicoil.

 

[Steptoe]

He has noticed this earthing bolt has a hole/vent through it.
Question is, is the vented bolt crucial, or can we replace with an ordinary bolt, and what's the best method to rectify this stripped thread.
I've suggested relocating the earth lead to another gearbox bolt, but I think he wants to keep it original.
Access to this bolt area is restricted by the frame, so any tips please.

The small hole through the earth bolt acts as the gearbox breather.

 

[kirby morgan]

Superb fast response and excellent advice, what a great forum.
Might have known Steptoe would know, good to see your feeling better, many thanks.

Sent from my GT-P5110 using Tapatalk

 

[Voyager]

I'm not entirely sure about a 'pre 1980' gearbox but if its similar to the later ones then that bolt also holds the speedometer cable in.

The vented bolt is used on the Paralever models, where the drive shaft runs dry. The earlier Monolever models allowed the gearbox to vent into the leg - which has oil in it. I'm not sure about a 'pre-1980' model - I assume its a 'Twinshock' (?). If it has oil in the leg with a breather on top then you won't need a vented bolt.

The gearboxes have different output shaft oil seals depending on how they vent - Monos have the lip seals facing out whereas Paras have them facing in.

The bolt is 6mm, but it has the end turned down to form a spigot, so don't just go banging a conventional bolt in there.

Ideally remove the gearbox and helicoil the stripped thread. I'll need to check whether the thread extends past the speedo cable hole in which case two Helicoil inserts might be required (plus a bit of tidying-up).

Bob.

 

[Rob Farmer]

You can buy a shorter helicoil that does the job nicely.

The usual temporary repair for this issue is to take the next 6mm cap head out of the top of the gearbox and replace it with a 6mm threaded stud and then fit two or three 6mm nuts on before fitting the earth lead followed by another 6mm nut. There are more elegant solutions but the one suggested uses parts you will have laying about.

 

[kirby morgan]

Temporary repair done as suggested by utilising the adjacent gearbox bolt ad a new earthing location. Vented bolt now locates into original hole and tightens up. Bike done 170 miles today with no issues.
Thanks all for advice and expertise.

 

[Rambozo]

The vented bolt is used on the Paralever models, where the drive shaft runs dry. The earlier Monolever models allowed the gearbox to vent into the leg - which has oil in it. I'm not sure about a 'pre-1980' model - I assume its a 'Twinshock' (?). If it has oil in the leg with a breather on top then you won't need a vented bolt.




Bob.

Twin shocks and monos both need a vented bolt as well