Author Topic: Gearbox mount bolts quandary? A7 Plunger  (Read 1466 times)

Offline RoadRunner

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Gearbox mount bolts quandary? A7 Plunger
« on: 12.09. 2010 20:03 »
Even'in all,

I'm so close but so far! The engine is in but... I've only just found out that two threads in the gearbox are not that good - these are located beneath the g/box at the rear where the frame bolts to the g/box. The original threads should be 5/16" 18TPI and the lock wired bolts are fairly short (3/4" max) - at sometime they have been drilled to 3/8" 16TPI and just over an inch long in length; one thread is okay-ish but a little loose, the other only locates on the last 2/3 turns.

The quandary I have is whether to helicoil or bodge (not really my scene).
BUT I really don't (I mean I really don't) want to take the engine out as a lump to get them helicoiled which will be difficult without stripping down the g/box (plus blinking heavy); on the other hand I don't want to bodge it excessively either.

So my question is just how much load is placed on these two bolts especially as the originals were so small and short? And will the existing poor fitting bolts be enough with a bit of toctite on?

I probably know what the answer will be (engine out *conf*), but I'm clutching at straws cos the last thing I want to do is attempt to lift the engine out, start stripping bits off and taking the paint off the frame. *eek* So if someone knows of a fix that may work I'd be grateful!

Once this cracked it won't be long before I can attempt to start it after 24 years!

Thanks in advance for any ideas;

RoadRunner



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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #1 on: 12.09. 2010 21:01 »
G'day RR,
                 If you really don't want to pull the lumb, I'd look at the liquid thread repair kits available today. I have one thats a two pack mix and a release agent. Clean the hole with carb cleaner and blow out, mix the two parts and put in hole (for those holes I'd lay bike on it's side). Put release agent on bolt and screw it in. Leave it in for 15 minutes then screw it out, let it cure over night. The packet said it's good for 125 ft/lb of torque on the bolts so should be quite ok for that application. After it's all done I'd shorten the bolts by 2 threads (or use washers) so the bolts don't bottom out when tightened.
Hope this helps.
Cheers.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #2 on: 13.09. 2010 23:24 »
Hi Muskrat,
Thanks for the suggestion - I've not heard of two part liquid metal with release agent - so the search is on. I'll let you know how I get on. Have a great day  *smiley4*

RR

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #3 on: 14.09. 2010 10:31 »
Just went down to shed. It's Permatex Stripped Thread Repair item #81668 says good for 150C and 128ft/lb.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline iansoady

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #4 on: 14.09. 2010 12:33 »
Personally, if it's the gearbox mounting studs, I'd say that wasn't nearly strong enough and you'd need a proper repair (helicoil, welding and retapping etc) as there'll be a lot of stress on them. If you look at the data sheet for the permatex product it states only 7 ft-lbs for 3/8" and 4 for 5/16". These kinds of things are OK for cosmetic repairs but not where there's any significant load on the part.
Ian.
1962 Golden Flash (arrived)
1955 Velo Viper/Venom (departed)
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Offline cotterpinkid

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #5 on: 14.09. 2010 16:51 »
Prior to its rebuild I ran my plunger for without realising it hadn't got any gearbox bolts (oh! the joys of a youthful uneducated BSA owner). Always thought it was some sort of elastic mounting that enabled the engine to move around *conf*. Needless to say it was one of the jobs on the rebuild list whilst the engine was out of the frame.
A10 GF Plunger
Velocette Venom

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #6 on: 14.09. 2010 20:31 »
Doh, after using the product many times Ian prompted me to read the instructions (if all else fails, read the instructions). I agree that a helicoil or welding up would be better, but without removing the motor it is an option.
To helicoil with the motor still in you would have to open up the frame holes to make the repair. Then make sleeves for the holes to bring back to size, as they are for locating as well as fixing.
I will now read ALL instructions before use (I still can't find the one for the WIFE).
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #7 on: 22.09. 2010 22:54 »
 *smile* Hi,
Thanks to all for those suggestions - finally bit the bullet and hauled the lump out, had the two holes helicoiled and the lump is now back in place!! Didn't fancy seeing the engine roaming around the frame *eek* and now fairly proficient at manoeuvring the engine in and out. Will post some pics later this month all being well.
Oh - I used plumber's foam pipe lagging to protect the frame which worked a treat.
 
Thank to all,
RR

Offline cotterpinkid

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #8 on: 23.09. 2010 20:57 »
RR,

Nice one. A bit of pain to do, but I bet you feel much better now? *smile*

Brian
A10 GF Plunger
Velocette Venom

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Re: Gearbox mount bolts quandary? A7 Plunger
« Reply #9 on: 23.09. 2010 21:07 »
Hi Brian,
Yep - I sure do feel better; I don't like bodging things but I'm no engineer either but the evil deed is now completed, also the top gearbox mount is much better than before  *smile*.

Your plunger looks good! One day.......

Thanks for the advice :-)
RR