AJS Model 30 re-bore

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Joined: Sat Aug 12, 2006 8:33 pm
Location: KENT UK

AJS Model 30 re-bore

Post by Miker5588 »

I am intending to have my 1957 model 30 re-bored to +20, and I have purchased a pair of pistons from the club spares scheme. The engineering firm which have agreed to do the re-bore have asked for the running clearances. Does anyone know what these will be?

Many thanks

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Joined: Wed Sep 01, 2004 1:00 am

Re: AJS Model 30 re-bore


Oh dear, here we go again! (No offence Mike), and I'm happy to repost my previous comments from past posts on this subject.
There have been regular complaints/questions on here about club pistons not being supplied with clearance data. Try a 'Search Forums' on 'Piston clearance' or 'Rebore clearance' and you'll find chapter and verse, pages and pages on this topic going back 10 years or more.
I have to admit that I don't buy club pistons any longer as 8 years ago when I asked JSL twice for data which was not included with the piston I was told "Any borer worth his salt will know" which in my frank opinion is unhelpful. (Politicians' euphemism for complete tosh).
There are a number of considerations, including the coefficient of expansion of the piston material and a significant difference between clearance required for air and water cooled engines.
FWIW I applied my own (?) rough and ready rule of thumb of 1.5 thou clearance per inch of bore diameter. Some would say that's a bit on the high side and it can produce piston slap in a cold engine but I'm not interested in posing with the quietest trailered bike at the show. I'd rather it slapped bit when cold, maybe used a bit of oil, but took me reliably o'er hill and dale.

(...........and RELAX..... :) )
I poke badgers with spoons.....from 2 metres.
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Les P H
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Re: AJS Model 30 re-bore

Post by Les P H »

I have emailed Gandini pistons and asked for info...bet they don't reply....
I think it is absolutely absurd that a manufacturer does not at least suggest a clearance.....a bit like selling a computer disc and saying you'll have to guess the security code. Another thing thing that baffles me completely is why modern pistons are heavier than the originals.....it just does not make any sense at all....Surely with modern metals, design software and ultra high precision computerised machinery you would imagine a piston of the exact same weight could be made quite easily???....Have the manufacturers even been told the original weight?...JP pistons are nearly 50% heavier ....crazy.

Will let you know if Gandini does reply.

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saltbox alf
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Re: AJS Model 30 re-bore

Post by saltbox alf »

As I understand it, the pistons especially JP are all blanks and then machined to size, so that would be one issue for a diesel tractor to an racing Itom.
Allons-y, amis de Matchless et AJS
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Re: AJS Model 30 re-bore

Post by g5wqian »

hi Mike , i guess this is probably a bit late in reply to help you much but i have just repaired a model 30 i now own that was previously rebored + .040" and uses gandini GPM pistons as you can buy from the club spares .

it had seized at some point on the nearside cylinder and although still ran it was knocking when it got hot .

upon inspection i found that the bore was bored to dead size and when measuring with a bore gauge it showed only +.001" maximum clearance .

i have stripped it down and bought a new set of pistons/rings from the club this week and put them in it .

i spent a few evenings honing the bores out by hand with my old delapena hone and have taken it out to average +.005" to +.006" of clearance as best i can get with the gear i have got .

i have reassembled it all with the new pistons etc , albeit i had real big problems getting the club spares composite head gaskets to go over the 76mm spigots [ someone needs to check the gasket manufacture dimension quality i think ] because the copper ring was too small , but eventually i got it sorted by putting them upside down on another spare barrel which they did fit and then let them sit for a while before removing and putting onto the bikes barrels .

i have subsequently ordered some solid copper head gaskets and i think will stay clear of the composite ones in future and will change to the solids once i have run the bike in .

i ran the ajs up today , second kick and a cloud of smoke or more later and it ran fine .

i rode it for 40 miles at 40-45mph and no problems around the local country roads .

i did note that the old seized piston had a brown heat stain inside at the rear , and so was running tight with the lack of clearance in the first instance .

i cannot say how my machine will fair now in the long term as its early days , but i am one who is glad of the technical advice i have read on the forum here relating to running clearances and i have applied them on this engine here .

i had a bit of smoke to start with but i dont expect it to be clear until the rings have bedded in later on .

prior to starting it i checked compression pressure in both cylinders at same time and had 130psi , which was almost same as the good cylinder had before i stripped it down .

im currently running on 30 grade oil .

the 73mm +.040 GPM pistons weighed 365grams each including pin and rings .

fingers crossed i will be loose from now on !! , and run it in over the xmas break .

cheers to anyone for any of youre advice i have read as its been of great help to me .

all the best
ian [swindon]
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