650 Barrels on a 500

[atreyu]

With my '55 G9 requiring an engine build this winter I would like to do abit of fettling to help it keep up with modern traffic!

In short is it possible to fit 650 barrels/heads on a 500 bottom end? Apart from guessing the crankcase mouth would need opening up, are the stud spacings the same? I've searched but cant really find any definite answers.

Many thanks

[moley]

The cranks are different on the 500 and 650, the shaft where the conrod bolts to is in a lower position. cheers

[wilko]

The crankcases changed when the 600 came out, so i think they changed the stud pattern. If you did manage to fit them you would end up with a 600 having same bore and stroke.

[Groily]

According to the cases from a '54 500 and a '61 650 sitting in the shed, stud spacing is as follows (quick look using a slide gauge, not definitive but tells you what you need to know - nb the studs don't form a square, not by a long shot):

500cc:71mm apart 'across' the engine, 65-66mm apart fore and aft 'long side' (outer) and 53mm shorter (inner) side
650cc: 73mm across, 70mm and 57mm fore and aft

So quite a difference. Enough to make for serious re-engineering.

500 and 600 have the same stroke but not the same cases AFAIK
650 top end is all different (and 650 cases are way different from old 500s), and also significantly differ in the cylinder department between the first year or so and subsequent. (Plus all manner of minor changes year on year for all models). Don't know if the crankcase throats on 600s and 650s are the same or not as haven't got a 600 to look at - others will know.

The only common bits across all sizes as far as I can see are most bearings/bushes, the rods and maybe some minor bits in the rocker gear department, camshafts, timing pinions (not always the idlers though as the pin they turn on changed diameter some point/for some models). Then there are the oiling system differences to think about as well, etc etc etc if going 'mix and match'. Which I wouldn't.

A 66mm (standard 500) cylinder will bore right out to 70mm-odd if you can find pistons to suit, but the cylinder spigots are extremely thin-walled at that stage, plus the fact that the pistons are wider than the combustion space and may kiss the heads. Don't ask me how I know. The thinness is maybe manageable, as the spigot is long and well-supported, but I got cold feet after spending ages making a 70+mm bore 500 work (and boy it was fast and revvy), and have mine back to 66mm now.

I'd say your graft is a complete 'No Can Do' from the start. Someone did shorten some 650 barrels and create a new surface on the tops a while back if I recall, but you're talking a fair bit of machining and welding if you want to try that sort of thing. Plus the fact you haven't got a 'proper' engine at the end of it, if that matters.

A full engine swap would be easier if you have one; otherwise why not just stick with the 500, sort it out properly for sale, and then get a Mod 31/G12?

[zwarts]

Having spent a bit of time in the workshop, I can confirm Bill's measurements. If your crankcases were 1956 or later, you could have fitted 650 barrels, heads and pistons but that would still have left a problem with the c.r. as the 500 crank would not push them far enough up the barrels (there is a way round this too, but things get a bit complicated and you have to question the amount of effort involved for the improvemet gained). The 500's can be made to go quite well, I can send you photos of 2 absolute crackers being raced (in Holland) if you want to see which way to go, so this is the easiest option. I can also advise on cams and valve timing, having spent the last 30 years experimenting with AMC twins !
Bob.

[atreyu]

Many thanks for all the replys, It's all interesting stuff and as when I tried to get an understanding of these bikes before, very confusing with all the different suttle changes throughout the years.

Bob, Would it be possible for my dad to have a chat with you about what we can do? If so could you PM/Email me a contact number? I'd love to see the photos, I dont know if you can get my email from my profile on here?

[wilko]

You could use special pistons with lower gudgeon pin holes to bring back your compression. My brain doesn't allow me to work out the difference in height. 7mm or 14mm difference?

[Don Madden]

You can fit 600cc barrels, pistons & heads for considerable improvement & there are a lot of the bits about, at least in the US. I would re-balance the crankshaft or replace it with a 600 item. As most new pistons are somewhat heavier, it would likely need re-balance anyway. Cheers, Don.

[zwarts]

I don't think you can fit 600cc barrels to a 1955 crankcase without opening up the crankcase mouth and relocating the head/barrel studs.
Bob.

[Don Madden]

You are right, Robert. Overlooked that the engine in question was pre-1956. Cheers, Don.