Re: Matchless 350 G5 piston and rings.

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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ajscomboman
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Re: Matchless 350 G5 piston and rings.

Post by ajscomboman »

Rangy65 wrote:No problem Rob but it has to be modified its not a direct replacement.
It is almost, the mod merely involves easing the edge of the skirt by 1/4" to ensure it clears the flywheel. A very quick job and nothing involving the crown or ring groves etc.
Andy1730
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Re: Matchless 350 G5 piston and rings.

Post by Andy1730 »

Hi all ..I'm having a problem with the build found out it needed new valves so ordered a pair for a 1960 350 G5 and what I had delivered was totally different from what was taken out of the head the valve stems were lot thicker and the valve heads were larger diameter now im not sure if I have a 350 or if it has had a different top end fitted..The valves I removed are...Inlet shaft 7.10 mm head size 41mm and it's 113mm long..exhaust valve is..shaft 7.40mm..head size is 31mm and it's 103mm long..I have measured the stroke and it's 79-80mm..so is this consistent with a 350 or if not what model etc..these are approx sizes so apologies if I'm a bit out..Also I was given two gearboxes one is smashed inside and the other is missing 3 gears and the selector forks so I'm needing a gearbox..Many thanks Andy
Last edited by Andy1730 on Tue Sep 22, 2020 10:49 pm, edited 1 time in total.
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clanger9
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Re: Matchless 350 G5 piston and rings.

Post by clanger9 »

Hi Andy,

Welcome to the wonderful world of Lightweight parts.

Your best source of info is this (scrappy) scan of a later parts list that shows most of the variations:
http://archives.jampot.dk/book/Spares_l ... o_1966.pdf

This is the "Dead Sea Scroll" of Lightweight spares. Rob has been asking for years if anyone has a better copy of this list, but nothing has turned up so far...

There are two exhaust valve types for the 350: the early one (042033) is the same as the 250 and has a narrow stem. The later one (042868) is shared with the 250CS and CSR and has a fatter stem. Both are the same diameter.

There is only one type of inlet valve for a 350: 044002. This is a bigger diameter and is shared with the later 250CSR and the head has a 1 1/8" inlet tract. 250 (non-CSR) heads have a smaller inlet valve (044032) and 1 1/16" inlet tract.

The two head variants (i.e. "250" and "350/250CSR") are physically the same size, so it is possible for an engine to get mixed up over the years.

Lots of good info in this thread: http://www.jampot.com/forum/viewtopic.php?f=16&t=23809

I probably have the gearbox parts you need, so send me a PM.
1989 Moto Morini Dart 350
1993 Ducati 750SS
Red Gauntlet
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Re: Matchless 350 G5 piston and rings.

Post by Red Gauntlet »

The '59 650 piston that fits the G5, do the over bores match i.e is a +20 G5 the same as the +20 650 piston? I gather that the skirt has to be shortened by about 1/4" presumably the balance will be about the same once modded? I would imagine the carb might require a small re-jet as well.
Max
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Re: Matchless 350 G5 piston and rings.

Post by ajscomboman »

Mighty Max wrote:The '59 650 piston that fits the G5, do the over bores match i.e is a +20 G5 the same as the +20 650 piston? I gather that the skirt has to be shortened by about 1/4" presumably the balance will be about the same once modded? I would imagine the carb might require a small re-jet as well.
Max
Yep they size up the same, the rear skirt of the piston needs to be shortened to clear the flywheels as the crank and head are off centre. Gives higher compression but the jetting should remain standard, I don't recall having to change mine.
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Re: Re: Matchless 350 G5 piston and rings.

Post by Red Gauntlet »

Thanks for that, I'll give it a go, I seem to remember the bore being forwards of the crank centre is called Letanche? Not had the engine apart yet, are the flywheels cast or steel on the G5, I seem to remember on the G2 fracturing many pairs before changing to the steel ones of the CSR, the G2CSR engine I have has 3 main bearings a short bush and a quill feed to the crank end.
Last edited by Red Gauntlet on Fri Feb 26, 2021 8:44 pm, edited 1 time in total.
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Re: Re: Matchless 350 G5 piston and rings.

Post by clanger9 »

1989 Moto Morini Dart 350
1993 Ducati 750SS
Red Gauntlet
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Re: Re: Matchless 350 G5 piston and rings.

Post by Red Gauntlet »

Does the ignition timing remain the same as it does vary with the G2 compression changes?
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Re: Matchless 350 G5 piston and rings.

Post by Red Gauntlet »

OK, so I have just bought a pair of '59 650 pistons and the differences are marked, the height from the top of the gudgeon pin to the crown on the G5 350 piston is 35mm whereas the 59' 650 piston is only 27.38mm, there is no way that these are compatible, the 650 piston would result in much lower compression thereby defeating the object of the exercise, The piston that is shown in the picture posted on page 1 of this thread shows a 650 piston that appears different from this one I have in my hand. Could someone who knows please check and furnish me with the correct data, unless what I have is not a '59 type 650 piston.
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Re: Matchless 350 G5 piston and rings.

Post by ajscomboman »

I'd say that is a 600 piston, the gudgeon pin looks to be too close to the top edge of piston. Not unknown for Gandini to put the wrong piston in the box.
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