HC Pistons or not ???

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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HC Pistons or not ???

Postby Flick5848 » Thu Dec 07, 2017 6:36 pm

I am about to embark on a rebuild of my 1960 Model 31 and thought it be a good opportunity to fit the high compression pistons the club offers.

Is this a good idea or do I need to make other changes ????

Thanks in advance

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Re: HC Pistons or not ???

Postby g5wqian » Wed Dec 13, 2017 6:55 am

i have a model 30 ajs and standard GPM pistons , but when i fitted solid copper head gaskets which happened to be too thin , i got a higher compression but this caused the engine to pink [knock] under load in high gear on inclines .

when i fixed the issue and put composite gaskets back on , the compression was slightly lower by around 10lb in pressure but it does not pink now .

i am running a magneto and manual advance , i dont know what the distributor ignitions with auto advance are compared to the mag in total advance used but if you use high compression pistons you might need to alter the timing to reduce the amount of total advance in the ignition if it knocks at all .

i tend to see a few posts from triumph owners who have holed pistons and these engines seem to use high compressions , higher than amc engines though , but it does show that the combination of low octane fuel and high compression and ignition advance are a bit of a headache in namy cases these days .

hopefully youll make a decision and go with it and it works ok for you .

my engine is only the 7.5-1 type and i think the g12 can use 8.5-1 pistons as an alternative , anything over 9-1 is real iffy if using pump petrol these days .

i blew an engine up in around 1987 when the govt reduced the octane rating and lead content almost over night by 25% of petrol , and had 12-1 pistons in which were fine before that but not after , i went back to 9-1 pistons and altered the total advance down by 4 degs and it was fine again , flat out no problems .

all the best
ian

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Re: HC Pistons or not ???

Postby Rob Harknett » Wed Dec 13, 2017 10:38 am

If your bike is running OK why change it. If you really need to go faster change it. Something I have done. It ended in me having to pay a months wages for an engine rebuild. I was only 17 at the time, lesson learned the hard way.

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Re: HC Pistons or not ???

Postby petert120r » Wed Dec 13, 2017 1:51 pm

By fitting 8.5 - 1 pistons instead of the standard 7.5 you're effectively upping the motor to CSR specs. The only tuning difference is a bigger main jet (450 from 390 if I recall correctly). Having rebuilt a number of both standard and CSR G12's in practice I suspect you would notice minimal actual improvement in performance unless you really intend to thrash it!

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Re: HC Pistons or not ???

Postby Flick5848 » Wed Dec 13, 2017 6:26 pm

Thanks for your replies

I bought the bike as a non runner and subsequently discovered it needs a rebore and crank regrind. Im not upset by this because I like the thought of a freshly rebuild engine and will still be on the right side with the £'s which keeps the wife happy.

Im not looking to thrash it or produce a racing bike but thought making it CSR specs would make it slightly more enjoyable. After all, they gave the CSR a higher compression ratio for a reason but it will have a nice easy life with me with the odd squirt of excitement.

Ive bitten the bullet and ordered the pistons and intend to make other adjustments inline with them. Carb etc.

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Re: HC Pistons or not ???

Postby dave16mct » Thu Dec 14, 2017 1:55 pm

Hi Chris,
where abouts in Lancs are you? We have a very lively, well supported section in the North West. We meet at Charnock Richard Football Club, just off the A49, on the first thursday of the month at around 8pm. You would be made very welcome. Cheers Dave.
Website: ajsmatchlessnw.org.net

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Re: HC Pistons or not ???

Postby Flick5848 » Fri Dec 15, 2017 8:24 pm

Im in Rossendale so roughly 35 minutes away from the meeting point

I have only owned the bike for a few weeks and have every intention of coming to have a look at you all once the bike is road worthy.

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