Seizure!

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
Red Gauntlet
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Seizure!

Post by Red Gauntlet »

In all the years of running lightweights, I have never had one heat seize, this engine three times now, it has done a couple of thousand miles on the bore, a new oil pump and big end, so well run in but just keeps nipping up. Not going fast just plodding around at 50mph. I am beginning to think that there is oil starvation to the big end as it is showing signs of seizure right in line with the con rod. Anyone else come across this before?
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Groily
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Re: Seizure!

Post by Groily »

Something is clearly not right to have had this happen with such monotonous regularity.

Can we assume the bloke that did the rebore had the piston to hand and ensured the correct skirt clearance, ring gaps and so on for the bits being used? (Given that some replacement pistons may requires more generous clearance than original?)
And that the mixture's not weak?

Does the con rod itself show any signs of discolouration or notchiness in the big end area due to heat / lack of lube? Hoping not . . . And are there any other signs of lack of oil around the place - top end, timing side, wherever?

To my eyes, it looks like a fairly 'normal' light seizure and I'd re-measure the piston skirt clearances as a first move, as the seizure is on the thrust faces where load is greatest, and make sure the ring gaps aren't too tight. 3 thou per inch of bore is the usual yardstick, but a fraction more wouldn't hurt.

Hard to say from looking down the cylinder, but maybe those scuffs will hone out, or near enough, and here's hoping the piston will clean up OK if it's smeared on any of the lands or above the top compression ring. They often will, I've found.
SPRIDDLER
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Re: Seizure!

Post by SPRIDDLER »

Groily wrote: Thu Apr 14, 2022 9:12 am To my eyes, it looks like a fairly 'normal' light seizure and I'd re-measure the piston skirt clearances as a first move, as the seizure is on the thrust faces where load is greatest, and make sure the ring gaps aren't too tight. 3 thou per inch of bore is the usual yardstick, but a fraction more wouldn't hurt.
I concur with all of Groily's comments.
My rule of thumb for piston to bore clearance is 1.5 thou per inch of bore (and ring gaps the same as Groiy's @ 3 thou per inch of bore).
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Tolly
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Re: Seizure!

Post by Tolly »

The one thing I can add is if the timing side shaft and bush is worn then the drive worm wheel to the pump disconnects and the oil pressure fails or reduces drasticly. When fitting a new big end did you replace the timing side bush and shaft?
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MikeM.
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Re: Seizure!

Post by MikeM. »

Did you assemble the engine yourself and if you did was the gudgeon pin a nice smooth fit in the small end bush, it may have minute high spots which could cause the engine to nip up even though it’s run in. This happened to me on my first bike and I had to take the high spots off with a small scraper, no problems after that.
Red Gauntlet
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Re: Seizure!

Post by Red Gauntlet »

Yes I assembled the engine, but I have built hundreds of engines air and water cooled in my time in the motor trade and I don't know how many G2 CSR's (Crank Snaps Regularly) engines. The timing was off by a degree or two but not so much that would upset it, although it has bob weights they are free enough, never had a problem with them in the past, everything is correct, spark plug is a nice colour, the first barrel and piston had actually done Isle of Man and back so no problem with those on another engine, that's why I used them saving running in time, the second set was from an engine that had done 100 miles, again with an original piston, this has now covered about 1000 miles approx. The crank has a new set of shafts and big end, new oil pump, timing bush and clean oil, piston and ring clearances are correct to spec. The first barrel and piston seized one day after I had done about 30 miles so I changed it thinking it was an isolated incident the second barrel and piston was when I had done about 50 miles non stop, then another 30 miles on again, I pulled the clutch and then rolling restarted almost immediately and carried on with out problem. I will have to tear it to pieces and look carefully at everything, I have not had a G5 before and it does seem a little sluggish over 50mph but maybe they are all like that, I generally don't push it too hard anyway. In between seizes I have done about 2000 miles. The first barrel honed out ok and I have just got a new piston (JP) for it which is correct clearance according to the 1.5 thou per inch rule so I will try that but only once I have found this issue. Oil gets around ok with the usual bubbly spurt from the return, but it is a pretty dry engine and does not have any oil leaks apart from the usual weep at the rocker cover gasket. One thing I have noticed with this engine it does get pretty rattly after about 30 or so miles and it's not the tappets.
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Groily
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Re: Seizure!

Post by Groily »

It's certainly a puzzle, and those smears don't look pretty on the piston. I was thinking they might be higher up than 'all south of the oil ring'.

It's strange that various other bits have run together on other engines but don't like it on this one . . . Is the gudgeon pin a correct fit in both rod and piston? Don't know if G5s have bushed small ends or not, but I'd expect the pin to pass through a warmed piston and room temp rod with firm sliding pressure, and the piston to pivot smoothly when assembled.

1.5 thou per inch would give you 4.25 thou clearance on a 72mm G5 standard bore - same as on the 650 twins in fact. I think (could be wrong) that if you asked T&L Engineering or others who use JP pistons regularly that they'd go for an additional thou or thou 'n a 'alf on the skirt clearance as they're prone, people say, to needing it. Worth getting clarification before risking any more expensive bits, maybe? I'm sure they need more than originals, and probably than some other replacement brands too.

Doesn't sound as if there is an obvious oil problem is it's returning normally, but is there something about the barrel lube that isn't right? I don't know a darn thing about the G5, but is there supposed to be any sort of splash or spray lube from down below that isn't hitting its target?

Your comment about excess rattly noises when hot is a bit worrying, even if unrelated to the present problem . . ; if not tappets . . . then ? ? ? Hope it's not a loose valve seat or something. Of course, one man"s death rattle is another's 'normal', but if it's noticeably more rackety than other similar machines . . .

I think you're right - it probably needs to come apart again, to try to find what the jinx is. Just doing the same thing over and over is a recipe for an empty pocket and a major sense of humour failure.
Red Gauntlet
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Re: Seizure!

Post by Red Gauntlet »

Very careful study of some of the parts, the piston skirt clearance to bore is the recommended 4 thou or 0.1mm so 0.25 thou tight by your reckoning which is not a great deal, I could give the barrel a quick hone to relieve that gap a bit, however I did notice that the piston has slight wear lower left and upper right looking from the rear indicating a possible either bent rod or timing side bush problem, now this is being very picky and I doubt it has much influence as I have seen many engines far worse off than this and they still run fine but I am going to strip it down and look at that bushing, the big end, if the oil is not pumped to it will syphon oil anyway so unless that oilway is blocked in the crank which I doubt then all will be ok in that area too. Still have not really found a smoking gun though. The rattling almost sounds like piston slap...but I suppose it could be a timing side bush? I just measured the weight of the JP piston 420g as opposed to 360g for the Hepolite.
Groily
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Re: Seizure!

Post by Groily »

Ah, so I s'pose there could be something then if there actually turns out to be a problem down below - no doubt all will be revealed. A bent rod certainly wouldn't help, 'tis true.

But I continue to worry that 4 thou is too tight on a JP at 72mm. Tales - whether bar-room myth or reality - of JPs nipping up for lack of clearance are fairly common.

'Recommended' per book on many AMC engines - those fitted with wire-wounds - was tiny compared to what's needed with after-market stuff, and without having the necessary knowledge to be adamant, I'd say 4 thou would be OK for Hepolites or AEs, but norrenuff for these Aussie lumps.

The instruction book for the G5 in the Archives actually states an original recommended clearance of a very miserly 2.8 thou, at top of skirt measured at right angles to the gudgeon pin. (Where, I think, yours has seized in fact?). Definitely way too little for JPs.

Maybe someone else could chip in with actual experience here?

If the oil wasn't pumping, would there be any return at all, or would enough oil be sucked in to get the scavenge side to produce a reasonable return?
Not sure what noise to expect from a worn t/s bush - on pre-unit Beesas fitted with similar I haven't ever really heard anything much, maybe a rumbly sort of grumble, but in those cases you lose oil pressure on the left hand rod owing to leaks en route, with unpleasant consequences if left unaddressed because of the use of shell bearings.
If your oil goes from pump to timing side axle to bush and to b/end in a loop, then wear there could perhaps cause grief. The book says the cylinder gets some splash, from what oozes out at the b/end I suppose. (I hadn't realised until looking at the exploded pix of lightweight engines that they still used the plunger type pump from days of yore - I thought they were into gear pumps by then like on the later heavyweights.)

Good luck with the next round of inspection . . .
g80csp11
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Re: Seizure!

Post by g80csp11 »

not wanting to open a can of worms , but here goes . Ive only had JP on my 650CSR years ago , and despite running the clearance that they recommended and very careful running in for 1000 miles . It still seized on way to Norway ( Nipped up 5 times before I got home )
My advise would be min 5- 5.25 thou , as standard would be 4.25 thou ( 1.5 thou per inch of bore)

The other thing is to make sure the small end (gudgeon pin) will slide under lightly oiled conditions through the conrod bush . Again in the past sometimes JP piston pins compensate for worn rods ( who would ever design that in! )

If teh small end nips up it will seize the piston

http://archives.jampot.dk/technical/Pis ... 4-1963.pdf
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