Piston up grade

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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Piston up grade

Postby Martin.S » Thu Oct 25, 2018 8:26 am

650 Cylinder Head.JPG

650 Cylinder Head.JPG

650 Cylinder Head.JPG


Rob Currie whilst recovering from a nasty get off very kindly sent me these drawings he made when he was calculating his compression ratio for his 650 CSR.
The combustion shape/dimensions appear to be virtually the same as my G5 head.
As the bore is the same and the deck height is within 0.029" (according to Don Maddens piston identification drawings) of the G5 piston I see no reason why the 650 piston will not fit the 350 lightweights.
Of course the stroke is longer on the 350 so the compression ratio would need to be adjusted perhaps by skimming the piston, thicker copper head gaskets and or thicker base gaskets - I was thinking 9.5:1 would be optimum.
Proof they say is in eating the pudding so I'm on the lookout to beg, borrow or buy very cheaply a scruffy 650csr piston to see how it fits and measure the compression.
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Re: Piston up grade

Postby clanger9 » Sun Nov 04, 2018 9:51 pm

Apparently a 1959 650 piston will fit (with some modification). It will give a useful increase in compression.
viewtopic.php?t=15584&start=10#p135761
1966 AJS Model 14 250 CSR "cafe racer"
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Re: Piston up grade

Postby Martin.S » Mon Nov 05, 2018 9:59 pm

Yeh thanks Paul, I have read about using a '59 piston but the compression ratio is unknown and have you seen the shape of the piston?
I was thinking of seeking out a 1960 onwards 650 CSR piston with a flat top that would allow a faster flame path(apparently) or building up the combustion chamber with weld for a more modern shape but advice is needed here. But if anyone has a '59 or 1960 onward piston to donate, lend or sell very cheaply for compression measurements it would be most appreciated. ;)
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Re: Piston up grade

Postby clanger9 » Tue Nov 06, 2018 8:05 am

I see what you're thinking. I don't know of anyone managing to get 9.5:1 on a 350 engine (it's more like 8:1 with the 650 piston), but that would be a useful goal, especially if you can get there with a flat top piston. Tricky, given the deeply recessed combustion chamber...
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: Piston up grade

Postby Martin.S » Sun Feb 10, 2019 5:34 pm

I've bought myself a nice used post 1960 650cc piston. The gudgeon pin is the same size as the G5. The outer diamter of the crown contacts the head so would need reprofiling. I've taken measurement with some thin oil and I recon that it's about 9:1.
The only problem is that the valve cut outs are in the wrong place. Some people on t'net have welded up holes in two stroke pistons with success but many poo poo the idea. So I'm going to try and get some professional advice here. I could possibly use Lumiweld but not sure if it would stand the heat - how hot does a piston get?
G5 and G12csr pistons.jpg
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Re: Piston up grade

Postby Mick D » Sun Feb 10, 2019 6:11 pm

Hi

Assuming the non standard piston is on the right:

I'd be very wary of removing too much material, but if you're only creating a clearance then maybe OK.

I certainly wouldn't try to add material and especially not Lumiweld - better to sacrifice a little compression than the engine when the bond breaks.

Regards Mick

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Re: Piston up grade

Postby Martin.S » Sun Feb 10, 2019 6:26 pm

I made up a mock piston to see how much i'd have to remove and practice machining the valve pockets - not much came off to clear the head.

Mock up piston.jpg
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Re: Piston up grade

Postby Martin.S » Wed Feb 13, 2019 4:26 pm

Well I went over to see Steve at Piston Broke here in Bristol. A definite no no to welding pistons - he tried it way back when and had troubles with distortion and expansion problems developed. He didn't think much about changing the combustion chamber in the head either - the valve seats aren't concentric with guides - a lot of work to put them back in - Oh bum!
So maybe take out what's needed for the valves on the 1960 650 piston and as Mick said put up a little less compression with the cut outs that are already there or try the 1959 650 domed piston - anyone got one to try?

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Re: Piston up grade

Postby ajscomboman » Wed Feb 13, 2019 4:55 pm

Martin.S wrote:Well I went over to see Steve at Piston Broke here in Bristol. A definite no no to welding pistons - he tried it way back when and had troubles with distortion and expansion problems developed. He didn't think much about changing the combustion chamber in the head either - the valve seats aren't concentric with guides - a lot of work to put them back in - Oh bum!
So maybe take out what's needed for the valves on the 1960 650 piston and as Mick said put up a little less compression with the cut outs that are already there or try the 1959 650 domed piston - anyone got one to try?


I have a +20 that has been done. What bore size are you looking for STD i suppose? I have a few but not sure if I have any single ones only pairs. I'll have a look.

I hate to say, I told you so, but, I told you it wasn't worth trying to reinvent the wheel! I went through the same process 20 yrs ago and came to the conclusion a 59 piston was the easiest way to go as it doesn't require cutouts for valves etc and no other machining other than the skirt to clear the flywheels.

I do like the trial piston you knocked up though

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Re: Piston up grade

Postby Martin.S » Wed Feb 13, 2019 5:27 pm

My barrel is +.030". So whatever piston I end up using will have to be +.040". But for a trial fit anything that'll fit in the present bore as I want to measure the compression ratio.
I suppose I could alter my mock up piston to replocate a '59 one if the radius of the dome were known.
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