1946 G3 piston and rebore

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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John D
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Posts: 105
Joined: Mon Jan 01, 2001 12:00 am
Location: CORNWALL UK

1946 G3 piston and rebore

Post by John D »

I've put a bitsa bike back together which has a 46 G3 bottom end with an Alfin barrel and iron head. I was hoping that I would get enough go out of it to settle the whole thing in a bit before investing further. However, compression is poor and its a pig to start. I've no other single to compare it with but I reckon its going to need a rebore and full top end job. No doubt what I should have done in the first place but hey ho. Before I take the top end off can anyone clarify the position re getting hold of pistons and or rings? I've read a good number of posts but got a bit mixed up and then I have the different barrel. I should have taken more photos and measurements pre assembly but have the following in my notes.
Piston is AE 10465 +40 which I reckon is Hepolite.
I'm assuming the conrod length is the shorter one before they were lengthened in 47?
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Duncan
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Joined: Thu Jul 10, 2003 1:00 am
Location: HAMPSHIRE UK

Re: 1946 G3 piston and rebore

Post by Duncan »

Hi John

It looks like you already have a 1948-1955 piston 6.3:1: http://archives.jampot.dk/technical/Pis ... x-1962.pdf

This is what the different pistons look like: http://www.jampot.com/pic_display.asp?id=2796

There should be plenty of pistons out there I believe the club were having some more made 7.5:1, but these will use different (size) rings to the originals (in the box)and you may need to ream the small end a touch as the pin (in the box) is slightly larger.
John D
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Posts: 105
Joined: Mon Jan 01, 2001 12:00 am
Location: CORNWALL UK

Re: 1946 G3 piston and rebore

Post by John D »

Thanks for that Duncan.
That's what I was thinking but I'm wondering how a 48 onwards piston was able to be fitted on a 46 crank if the gudgeon pin was at a different height? I didn't think it would be possible. That is making the assumption the conrod is the shorter one. I guess if I take the head off and can find the lenghths for both rods I can measure it.
The bike runs and the original was a diy trials bike hence the alfin barrel and why it's proving difficult to fettle.
Will the 7.5:1 ratio cause any issues?
John D
Member
Posts: 105
Joined: Mon Jan 01, 2001 12:00 am
Location: CORNWALL UK

Re: 1946 G3 piston and rebore

Post by John D »

I've taken the top end down and would appreciate helpful comment on my assumptions. Also, this engine was part of a diy trials bike hence the mods I think.
1. Piston is 10465 plus 40. Rod is numbered 010311 (expected 010312 from 47 on engines) can't find this listed anywhere but assume it was used to accommodate the later piston with lower gudgeon pin?
2. A compression plate is fitted. Non listed for 350's but again assume it was related to trials use, the Alfin barrel and the change of piston/rod?
3. I can feel wear in the barrel at the end of piston travel but don't have kit to accurately measure. I have been recommended a man who could check it and rebore. I will need a plus 60 piston! Only ones I can find are at http://vintagemotorcyclepistons.co.uk and I think are JP. Any other suggestions?
4.A copper head gasket (less than .5mm) was refitted but the seal was not good, should I use the spares scheme composite one I have (thicker)?
5.The head is iron. No caps on valves, later ones used? I reground the valves but suspect the guides and wonder about the crude filing of spring cups. To clear the rocker box? but why necessary? I think I've found valves and guides on the spares scheme, but can't find springs should I need them (I will re-measure but think they were within limits.
Hopefully, I've attached some pictures!
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