The BSA A7-A10 Forum

Technical (Descriptive Topic Titles - Stay on Topic) => Frame => Topic started by: Rex on 12.02. 2018 11:08

Title: 1951 A7 gearbox mount
Post by: Rex on 12.02. 2018 11:08

On my 1951 A7 plunger the two bolts for the rear gearbox mount are 5/16" Whit, but the holes in the frame are 3/8".
The holes are circular not worn and distorted, so has someone run a drill through them or did BSA make it like this?
It looks like it should have shouldered bolts fitted, but Draganfly's pic just shows common-or-garden hex-head bolts in this application. Any thoughts?
Thanks.

Title: Re: 1951 A7 gearbox mount
Post by: muskrat on 12.02. 2018 19:22

G'day Rex, good ? mate.
I seem to think from memory ( *whistle*) mine are 5/16". I recall the bolts are a bugga to get in. I'll look at my spare frame tonight.
Cheers

Title: Re: 1951 A7 gearbox mount
Post by: muskrat on 12.02. 2018 20:42

I'm back from clearing snakes out of bottom shed  *eek*.
My original frame has 3/8" holes so I removed the bolt from my working frame. It's 3/8" too. But both could have been "modified" in an earlier life.
Cheers

Title: Re: 1951 A7 gearbox mount
Post by: Rex on 12.02. 2018 21:11

Hmm...the two gearbox mounting bolts are deffo 5/16 on mine...doesn't seem a very positive location, and that's unlike BSA.
Thanks Musky.

Title: Re: 1951 A7 gearbox mount
Post by: duTch on 12.02. 2018 21:42

 The holes  in my frame are also 3/8", and fairly sure were that since I've had it since ~ '77, but I think all the boxes that I have are 5/16" mount holes, except the two that I've used and drilled and re-tapped to 3/8" 'cos the threads were shot (actually I think the one in use was already 3/8"- but shot, and I've used helicoils).

 I ordered a bunch of bits from Lightning Spares, and in the 'Frame -Fastenings' packet were two 5/16" bolts that I've never used.

Also, the bottom gearbox lugs were both fairly hammered from (everything) being loose and bouncing around. To remedy this, I filed 'em flat and square and have packed it up with a couple of stainless washers with the intention of one day making a single piece two-hole shim to do same.
 Hope that's a bit helpful.

Title: Re: 1951 A7 gearbox mount
Post by: muskrat on 12.02. 2018 22:18

The g/box bolt holes are 5/16" whit in my shells.
Cheers

Title: Re: 1951 A7 gearbox mount
Post by: duTch on 12.02. 2018 23:13

 Ok, curiousity overruled sensibility- it is part# 67-4028, and a quick innernets search gave me this result;

 https://www.baxtercycle.com/product/67-4028/ (https://www.baxtercycle.com/product/67-4028/)

Title: Re: 1951 A7 gearbox mount
Post by: Rex on 13.02. 2018 21:18

After most of the day spent getting the engine and gearbox mounted in the frame without damaging the enamelling, I can now appreciate why BSA specified 3/8 holes for the gearbox mounts; pragmatic engineering to overcome the manufacturing tolerances.
Two 5/16" clearance holes would have made it almost impossible.