1950 G3L rigid 350- 500 conversion?

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

Postby ajscomboman » Fri Dec 22, 2017 7:35 pm

Conrod is the same but flywheels are different (balance factor). Stud spacing is the same.

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Re: 1950 G3L rigid 350- 500 conversion?

Postby MikeM. » Fri Dec 22, 2017 9:16 pm

What about push rods and their tubes????

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Re: 1950 G3L rigid 350- 500 conversion?

Postby George Kaplan » Fri Dec 22, 2017 10:22 pm

I dont know too much about the details but I am reliably informed that a 500 top end will fit on a 350 bottom end without too much fuss. In fact I own such an engine although as yet I haven't fired it up. (see the last few posts here: viewtopic.php?f=17&t=20906&start=50

Re the pushrod tubes etc, why not get a parts list and cross check part numbers between 350 and 500 to see which items are common?

John

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Re: 1950 G3L rigid 350- 500 conversion?

Postby SPRIDDLER » Fri Dec 22, 2017 11:21 pm

George Kaplan wrote:I dont know too much about the details but I am reliably informed that a 500 top end will fit on a 350 bottom end without too much fuss.

Yes, it's a mod that has been carried out often, right from the days when the bikes were 'new'. However the OP's original question was whether the 350 head could be used on a 500 cylinder barrel.......

Matchymarty wrote: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?

Well the general conclusion seems to be that it probably could, but the benefit is debatable. Better to use a 500 head with the larger exhaust valve, plus the larger 500 carb and zorst.
The other main consideration is that of needing (or not) to rebalance the flywheels. Some haven't and seem to be happy with the result.
I poke badgers with spoons.

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Re: 1950 G3L rigid 350- 500 conversion?

Postby clive » Sat Dec 23, 2017 8:55 am

Matchymarty wrote: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?


Yes to all these questions except that the balance factor, as mention by Rob, is different for the 350 crank. This does not really cause a problem but may put peak vibrations at the speed you wish to travel at, in which case you will probably want to travel a bit quicker or a bit slower! You will certainly need a head steady that is properly clamped to the top frame rail ( the c clamp has sometimes distorted over the years and needs packing to ensure it grips the frame properly, loss of paint under it will be the evidence). Push rod tubes will need to be the correct ones for the year of the head. When mixing and matching different years you also need to ensure that the piston skirt clears the crank at the bottom of the stroke. I have tried a mix and match that resulted in it just touching as it rocked at the bottom of the stroke. Advice from Les at Russels was followed and a file applied to the skirt to increase the clearance. :oops: This may be particularly relevant if the title of your thread is correct and it involves a 1950 engine. The one I had this trouble on was a 1949 engine and I have a recollection that I was told the cranks at that period had a fractionally greater diameter. The bodge with the file caused no problems and very many miles were travelled on the piston as at the time it was my daily ride to work machine.
clive
if it ain't broke don't fix

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Re: 1950 G3L rigid 350- 500 conversion?

Postby clive » Sat Dec 23, 2017 9:14 am

SPRIDDLER wrote:Yes, it's a mod that has been carried out often, right from the days when the bikes were 'new'. However the OP's original question was whether the 350 head could be used on a 500 cylinder barrel.......


Keep up at the back of the class, things have moved on...... ;)

Matchymarty wrote: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?
clive
if it ain't broke don't fix

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Re: 1950 G3L rigid 350- 500 conversion?

Postby Pharisee » Sat Dec 23, 2017 10:14 am

With regard to push rod tubes... That will, as has already been said, depend on the head you decide to use. The earlier head as on the '55 AJS M18 below, uses the longer tubes which are outside the head casting for the first few fins. The head on the 350 G3 is a later version from around 1957 and uses the shorter push rod tubes.

18S_2340.JPG


18S_2850.JPG
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Re: 1950 G3L rigid 350- 500 conversion?

Postby Matchymarty » Sun Dec 24, 2017 9:23 am

Thank you all! Great information. Quite exciting notion.
So basically, it seems, that I can use a 350 bottom end, and drop on a 500 piston, barrel, head and suitable carb, and I'm away. Great news.

Cheers for all the information. Much appreciated.

For reference, if I do it, I'd be using a '50 350 bottom end and '50 500 top end.

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Re: 1950 G3L rigid 350- 500 conversion?

Postby George Kaplan » Tue Dec 26, 2017 12:31 pm

I got Bacons book on the recommendation from this site and it states that in 1948 from engine number 8000 both the 350 and 500 heavyweight singles used the same crankcases.

John

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