Seizure!

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
Red Gauntlet
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Location: Dorset UK

Re: Seizure!

Post by Red Gauntlet »

Ok with the engine stripped down now and everything checked, oil is getting around so no problem, piston to bore clearance was 4 thou, however the timing was found to be a considerable distance advanced (the bobweight taper was loose, well not tight anyway) which would cause overheating and running issues, the small end was tight so maybe part cause or possibly a result of the problem, I think maybe a combination of too far advanced timing and small end tightness causing overheating. Anyway here goes with a new piston, freshly honed bore 5 thou clearance instead of 4 and fresh oil, timing reset to 1/4" btdc bob weights fully open, I wonder what the timing is with the bob weights closed maybe tdc like some other machines, I will have to check that on reassembly before the head goes on.
The old piston cleaned up ok, so I will use that instead of the new one, done that a few times before, should be good, it was only a little scuff so careful dressing with a new paring file and 220 grit emery soon sorted it out, checked for high spots and all is well. So back together tomorrow.
Interestingly the JP rings are thicker and weigh in at 31g the Hepolite and Gandini ones 26g & 22g respectively, the Hepolite piston is lighter at 264g and the JP and Gandini both weigh the same at 303g, gudgeon pins original Hepolite is 79g, JP is 86 and Gandini is 68g. Not that the heavier parts should make much difference, you halve the extra weight for the balance.
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GSAX1
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Re: Seizure!

Post by GSAX1 »

The JP pistons that came with my '55 model 20 siezed regurarly under high load, even though they rattled when cold due to more than enough clearance. Finally they siezed properly and scored the cylinders, like yours. I threw them in the bin and fitted a pair of original BHB wire-wound pistons with very tight AMC factory tolerances. Never had any problems with siezures after that, and that is some 15 years ago.

The JP pistons were crap.

Your problem may be piston related also.
Groily
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Location: NORMANDIE FRANCE

Re: Seizure!

Post by Groily »

Not sure about JP pistons being 'crap' from my own 20K miles with +20 8.5:1s in my 650.
I had read all the comments and was nervous I have to admit - but I didn't have much choice at the time.

They were supplied by, and the rebores were done by, T&L Engineering. The bits all went together well with generous skirt clearances although I can't remember exactly how much.They didn't seize when new, haven't seized since - and they don't slap when cold. Am I just lucky? Maybe - that engine is given a good few beans up the gears with a new Club crank and Thunder rods etc and I've been happier than I expected to be!

Haven't used them in my Mod 20/G9 engines (one 'in', one ready-to-go 'spare'), which both have AE / Hepolites at the moment, although I do have a good set of 66mm wire-wounds handy for when one or other engine needs re-sleeving (after my riding life-time I hope!) And I have no piston experience whatsoever with singles. A tight small end will not have helped there, though, as Red Gauntlet says and one or two of us surmised could be relevant.

I guess maybe the 'best' pistons I've had in recent times were some +40 IMDs for an A10 BSA. I wish they made 'ours' too - their reputation for quality goes before them increasingly. But I wouldn't avoid JPs 'just because', as long as I had faith in the reborer. My faith in T&L remains unshaken . . . thankfully!
JEAN-NOEL
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Location: FRANCE

Re: Seizure!

Post by JEAN-NOEL »

Hello,
in the past (1996) I have experienced issues vith JP pistons :
-they were a lot heavier than the original
-the gudjon pin was oversized : heating the piston and small end, it was ok, but was tight when cold.
When I changed for GPM (GANDINI) in 2016, Derrick of T&L Engineering applied 6 thou for clearance, and the bike runs nicely (except misfires and a flat spot around 3000 RPM, but it is an other problem !)
My bike is a G12CSR with 73 mm pistons.
Kind regards.
Red Gauntlet
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Location: Dorset UK

Re: Seizure!

Post by Red Gauntlet »

Well that was subtle, I found the weird timing problem, the timing was too far advanced and I mean a lot 0.6", the reason being is that the 'new' pattern part points have a wider foot on them which overlaps the points cam to the outside, this overlap of the foot contacts the washer of the bolt that holds the bob weight assembly so instead of opening on the cam it was opening on the washer. Simple fix just file away the points foot on the outer edge enough to clear said washer.
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Groily
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Re: Seizure!

Post by Groily »

Just to say - whether it's a magneto or battery & coil ignition - never replace the points, contact breaker backplate or any other important part without checking the timing carefully afterwards. It will nearly always be affected. Sometimes just because the heel on the new opening point is unworn and they open earlier, thus advancing the timing; sometimes because of some weird thing like here, where the washer needed to be smaller o.d. or the heel dressed. Just setting the gap to whatever obviously doesn't fix it in these cases. (Same applies if fitting used bits, as the wear on the heel is never the same as on what came off.)

A half inch + variation, if talking piston position BTDC . . . . wow. We're in 'hole the piston' territory with that sort of error.
Red Gauntlet
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Re: Seizure!

Post by Red Gauntlet »

Yes, the crazy thing was though, the points were opening in the correct place as well as earlier which I failed to notice right away. When you time it up you set the points to open 1/4" btdc when you place a screwdriver in the bob weights, this was beyond where the washer was opening the points the first time if you see what I mean.
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