What top end?

Information relating to the Matchless G12 or AJS Model 31 650cc twin
trophy
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What top end?

Post by trophy »

Hi all. Trying to unravel a problem and I hope someone cleverer than I can help. My 1962 AJS Model 31 (engine number says 1960) has flat topped pistons, +.020" with number 15762 stamped on top. My cylinder heads have 026053 and 026054 cast in them. These numbers do not correspond with the spares catalogue I have for my model. How can I identify my barrel's as being for the '31--or could they be for the 30? I hope I'm wrong but I am beginning to suspect that the previous owner has fitted the top end of the model 30 onto the 31 crankcase. Is this possible? I have measured the stroke at a tad under 73mm and the bore at 72.5mm. If it is the model 30 then I feel a right fool not picking up on it until now. :?:
I have trolled the archives for part numbers in the hope of sorting the problem but to no avail. Help anyone!!!!
Ride safe all.
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Stuoyb
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Re: What top end?

Post by Stuoyb »

The numbers on the castings are "casting numbers" and not the finished (machined) part numbers so you will not find them in any parts catalogue.
I'm sure some sage will come along with info. on the bore/stroke and pistons.
Last edited by Stuoyb on Sat Jan 24, 2015 8:01 am, edited 1 time in total.
dodger
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Re: What top end?

Post by dodger »

Flat top pistons are correct for a Model 31 for 1960 on machines.
Count the fins on your barrels, 7 for a Model 30 and 8 for a model 31.
If you look under the cylinder head around the exhaust port you should see some small fins going of at an angle, this will identify it as a post 1960 head.
Laurie
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Re: What top end?

Post by trophy »

dodger wrote:Flat top pistons are correct for a Model 31 for 1960 on machines.
Count the fins on your barrels, 7 for a Model 30 and 8 for a model 31.
If you look under the cylinder head around the exhaust port you should see some small fins going of at an angle, this will identify it as a post 1960 head.
Laurie
Hello Laurie. I've taken the heads off to take photos and to confirm the stroke more exact than via the spark plug hole. The stroke is 72.8mm confirmed and this is the Model 30 spec, not the Model 31 which is 79.35mm. This tells me I have a model 30 crank :headbang:
My cylinders have 8 fins which you say is for the Model 31 but what puzzles me is that at TDC the pistons are just proud of the top of the cylinders as you would expect. According to your thread they should be 6.5mm lower :?:
The cylinder heads do have the fins as you describe but again this throws up another puzzle in that F W Neil's bible states that Models 30 and 31 top end parts cannot be interchanged. Model 30 was not marketed in 1960 whereas, if my barrels are Model 30 (pre 1960) and my heads are post 1960 then according to the FW they cannot go together :?
The deeper I dig the more confused I get. It doesn't bother me unduly that I have a 600 instead of a 650cc apart from the embarrassment of not sussing it out whilst I was restoring it. Should I want to put it back as a 650 would I have to replace barrels and heads as well as the correct crank?
I hope I can post the photos OK. Come on folks, put me out of my misery.
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Ride safe all.
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saltbox alf
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Re: What top end?

Post by saltbox alf »

definite 650 barrels and heads, now the question is, what con rods and crank? There has been talk of late triumph rods being used. So you will have to pop the barrels off and have a gander, the 650 rods AMC should have a cut out to clear the crank case as opposed to the 600 rods.
However if it goes well and don't need a rebore etc, don't painc!!!
Allons-y, amis de Matchless et AJS
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dave16mct
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Re: What top end?

Post by dave16mct »

Your barrells are 8 fin , so 650. Your heads have diagonal fins, so are '60-on (650 =1 5/8" exhaust port). The top spigots should be 78mm for '60 on. I didn't know you could fit 500/600 crank to 650? How does it run?
Cheers Dave.
Don Madden
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Re: What top end?

Post by Don Madden »

Note that there are two types of 8 fin barrels, 1959 G12/M31 had 3in diameter top spigot, with recess in heads to match, & 1960-up which had 1-1/16in for both cylinder & heads. And then there were the 1958 export 650cc twins that used the 7 fin barrels & heads same as the 600cc versions, many of which were made, mostly for Cooper Motors in Los Angeles. Cheers, Don. 8-)
dodger
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Re: What top end?

Post by dodger »

You may have High compression pistons fitted which would explain the apparent normal height of the piston protruding from the barrel, I have stood 7.5:1 flat top pistons beside 8.5:1 pistons and the 8.5:1 is taller to give it the higher compression.
AMC also produced 10.5:1 pistons which would be taller still.
Laurie
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Re: What top end?

Post by trophy »

I believe AMC twins con rods were the same size length-wise with the 650 rods having the chamfer to clear the cases, as mine have.
I don't think my pistons are high comp but as I mentioned earlier they are +.020 and stamped 15762, whatever that number means.
Considering how far my pistons protrude the cylinder at TDC If I fitted a 650 crank might the pistons/valves clash with the extra 3.25mm piston height? The heads have a very deep concave so the valves/pistons might not hit each other. What are your thought/opinions?
Forgot to mention earlier that my crank has the number R 742 stamped on the rim of the timing side flywheel. Does it mean anything?
Cylinder spigots are 3 and 1/16 (77.8mm) dia with the heads recessed to suit.
Everything is pointing to the fact I have a 600 crank fitted to a 650 top end and as I think the bikes compression is not the best (not bad but not good either, nowhere near other 650's I have owned) and I have only done 250 miles since putting it on the road. It has not run at all well but I have put this down to carburation problems which is ongoing with the makers. Seems like I will have to eat humble pie and let them know of my cc cock up. Wonder if it would help if I fitted 10.5:1 pistons.
I bought the bike just over 2 years ago. The previous owner passed away in 1988 and had been laid up since. The crank was in poor shape and I was unable to source a replacement. It required metal spraying to recover the big end journals and altogether cost me quite a few bob so I would like to keep it. Alternatively, if anyone knows of a sound 650 crank at a reasonable price please get in touch.
Keep your thoughts coming in.
Ride safe all.
trophy
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Re: What top end?

Post by trophy »

As shown in the photo in my previous post does anyone have any idea if the piston is protruding through the top of the cylinder not high enough, too low or just right.
Also has anyone got information on the difference in height from the gudgeon pin to the top of the piston for a standard piston and an 8.5 c/r one? Perhaps someone may have both in the box under the bench. Maybe someone might even have a used pair of 8.5 c/r pistons I can buy in an effort to up my compression a bit, maybe even a serviceable 650 alternator crank :!: I have posted wanted ads on the forum and in Jampot. If either items become available this would keep me out of mischief and from under the wife's feet next winter.
Ride safe all.
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