1950 G3L rigid 350- 500 conversion?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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1950 G3L rigid 350- 500 conversion?

Postby Matchymarty » Fri Dec 22, 2017 7:05 am

Hi all,
Just wondering if it is possible to 'bolt on' a 500 iron barrel and piston on to a G3L bottom end and head?

Contemplating a rigid G3L build but not sure how easy it is to convert to 500 for a bit more poke???

Cheers
Marty

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Re: 1950 G3L rigid 350- 500 conversion?

Postby clive » Fri Dec 22, 2017 8:33 am

The increased cc without changing the head and exhaust and carb did not seem to provide much more performance for a fellow section member. He returned it to 350.
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Re: 1950 G3L rigid 350- 500 conversion?

Postby Pharisee » Fri Dec 22, 2017 8:57 am

The combustion chamber volume in the 350cc head is a lot less than the 500cc head. That will raise the compression ratio considerably... maybe too much.

350-small.jpg

350cc head

500-small.jpg

500cc head
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Re: 1950 G3L rigid 350- 500 conversion?

Postby SPRIDDLER » Fri Dec 22, 2017 10:15 am

I wasn't aware that a 350 head (bore dia. 69mm) could be used with a 500 barrel (bore dia. 82.5mm).

Doesn't the 500 head have a larger inlet valve than the 350, or is that only on later heads?
If so, I would guess that some of the benefit of the extra cc would be lost by using a 350 head having the smaller inlet valve.
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Re: 1950 G3L rigid 350- 500 conversion?

Postby Mick D » Fri Dec 22, 2017 10:53 am

Hi

Wouldn't a 500 piston crown impact a 350 head?

Regards Mick

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Re: 1950 G3L rigid 350- 500 conversion?

Postby Pharisee » Fri Dec 22, 2017 1:57 pm

Mick D wrote:Hi

Wouldn't a 500 piston crown impact a 350 head?

Regards Mick


Depends on the 500cc piston. My '55 M18 has a flat top piston so no, it wouldn't but I have seen some that have a domed top and yes, they probably would.

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Re: 1950 G3L rigid 350- 500 conversion?

Postby cfaber » Fri Dec 22, 2017 4:09 pm

my '54 18s also has a flat top piston.

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Re: 1950 G3L rigid 350- 500 conversion?

Postby clive » Fri Dec 22, 2017 4:48 pm

SPRIDDLER wrote:I wasn't aware that a 350 head (bore dia. 69mm) could be used with a 500 barrel (bore dia. 82.5mm).

Doesn't the 500 head have a larger inlet valve than the 350, or is that only on later heads?
If so, I would guess that some of the benefit of the extra cc would be lost by using a 350 head having the smaller inlet valve.


The barrel has the same spacing for the head bolts and the recess in both heads will fit the 500 barrel. The inlet valves for both heads are the same size it's the exhaust valve that is bigger, hence the need to change both exhaust pipe and silencer if fitting the 500 head. Personally I would say the 350 cannot usefully be used with the 500 barrel, although it will fit. Compression much increased but poor exhaust breathing and on the intake side I think the difference in the carbs was more than just the jet sizes.
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Re: 1950 G3L rigid 350- 500 conversion?

Postby Pharisee » Fri Dec 22, 2017 4:56 pm

clive wrote:
SPRIDDLER wrote:I wasn't aware that a 350 head (bore dia. 69mm) could be used with a 500 barrel (bore dia. 82.5mm).

Doesn't the 500 head have a larger inlet valve than the 350, or is that only on later heads?
If so, I would guess that some of the benefit of the extra cc would be lost by using a 350 head having the smaller inlet valve.


The barrel has the same spacing for the head bolts and the recess in both heads will fit the 500 barrel. The inlet valves for both heads are the same size it's the exhaust valve that is bigger, hence the need to change both exhaust pipe and silencer if fitting the 500 head. Personally I would say the 350 cannot usefully be used with the 500 barrel, although it will fit. Compression much increased but poor exhaust breathing and on the intake side I think the difference in the carbs was more than just the jet sizes.


The carb on my '54 350cc G3 is 1.1/16" diameter... On my '55 500cc M18 it's 1.5/32".

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Re: 1950 G3L rigid 350- 500 conversion?

Postby Matchymarty » Fri Dec 22, 2017 7:31 pm

Thank you all :)

So seeming that head would also need to be changed. See I may be able to source a frame, engine and gearbox, and oiltank. Barrel is has many broken fins, so would need replacement. Say I can find a 500 head and barrel, is the bottom end the same for both 350 and 500?
Is the stud spacing for barrels the same in the crankcase?
Are the flywheels the same ?
Is the conrod the same and little end?

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