lot of smoke after rebuild, Help Please

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Mick D
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Location: Aberdeenshire

Re: lot of smoke after rebuild, Help Please

Post by Mick D »

Hi

I'm struggling to see how this can be a 'rings' issue. I assume the bores have been re-bored and honed to suit the new pistons / rings. The upper cylinder will be at a higher pressure than the crankcase for 75% of it's operating time, (during the induction stroke it will be very slightly below crank case pressure), so for 75% of the time pressure is resisting the flow of crank case oil past the rings.
If the crank case breather system is functioning correctly I think I'd be looking at the upper cylinder lubrication system.

Regards Mick
Groily
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Re: lot of smoke after rebuild, Help Please

Post by Groily »

Yes; but without the heads on Maurizio reported in his first post that oil is seen coming UP past the pistons even at kickstart speeds. Which suggests a problem with oil control at the rings as the oil can't be descending from on high if there's nothing up there.

Without the heads on, turning the engine on the kickstart shouldn't produce puddles on top of the pistons.

Not to say that there isn't a top end over-oiling question or breathing problem too, of course, as the lubrication system is a bit of a minefield and the factory never quite seemed happy with it judging from the frequent tweaks they made - and sometimes reversed!
Mick D
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Re: lot of smoke after rebuild, Help Please

Post by Mick D »

Hi

It is a bit of a weird one isn't it - after all, when turning by the kick start where is the oil supply to the bottom of the piston to supply the leak that is becoming apparent? (The lower cylinder is splash fed isn't it?)
I can only assume it's residual oil clinging to the cylinder wall. In this condition the piston rings are not operating as designed so oil will not be excluded as efficiently as when running.

What do the wear marks on the barrels indicate? If they are newly honed it should be easy to identify an even wear pattern or any anomalies.

Regards Mick
maurizio66
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Re: lot of smoke after rebuild, Help Please

Post by maurizio66 »

robcurrie wrote:One more thing to check concerning oil build up in the sump - are the feed pipe and return pipe fitted to the correct pump and are the pumps fitted in the correct positions. The smaller pump is the feed and should be in the front and the larger pump is mounted at the rear and connects to the oil tank return.

There is a vent hole in the inner primary cover and is close to the hole where the alternator wires go through.

Rob C
Ciao Rob,

I found one your old post from 2011 where you said "I Purchased a set of BMW R60 / 2 - R69 / S piston rings from Ulis Motorradladen in Germany. They fit perfectly!" Do you have more information about the rings you used, such ring width and thickness (height) for all the rings and if they have worked well?

Thank you for your precious help,
Maurizio
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robcurrie
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Re: lot of smoke after rebuild, Help Please

Post by robcurrie »

Maurizio, I still had original pistons, but at some stage someone machined the ring grooves wider to take metric size rings. Original ring grooves were 1.6 mm for compression rings and 3.2 mm for the oil ring. Metric rings, like BMW, are 2 mm and 4mm. I had no choice but to find metric size rings, but you should be able to get good quality imperial size rings quite easily.

Rob C
JEAN-NOEL
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Re: lot of smoke after rebuild, Help Please

Post by JEAN-NOEL »

I think that AMOC PARTS should question JP about pistons rings, and especially to obtain warranties for the pistons which they have on the shelves, at least to have technical advice. I am amazed that in 2017 piston rings could be bad : 50 years ago we had very good rings from Goëtze, or Perfect Circle...
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clive
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Re: lot of smoke after rebuild, Help Please

Post by clive »

Groily wrote:Yes; but without the heads on Maurizio reported in his first post that oil is seen coming UP past the pistons even at kickstart speeds. Which suggests a problem with oil control at the rings as the oil can't be descending from on high if there's nothing up there.

Without the heads on, turning the engine on the kickstart shouldn't produce puddles on top of the pistons.

Not to say that there isn't a top end over-oiling question or breathing problem too, of course, as the lubrication system is a bit of a minefield and the factory never quite seemed happy with it judging from the frequent tweaks they made - and sometimes reversed!
I have reread this thread a couple of times and cannot find this reference to over oiling whilst kicking over the bike with the heads off. Was it in a different thread perhaps? If not I think the little oilway restrictors to the head might be the problem. They seemed to fit them some years and not others, do some of them put oil directly into the inlet valve guide? The lack of marking on the supplied rings probably means they are not tapered and therefore do not need to be fitted a specific way.
clive
if it ain't broke don't fix
maurizio66
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Re: lot of smoke after rebuild, Help Please

Post by maurizio66 »

Thank you Rob, my actual rings are 2 mm compression, 4 mm oil scraper as your from BMW, I just need to find how is the depht and if could fit in the JP groove.
I know that JP produce pistons but rings come from different manufacturers and probably happen that some stock are poor quality, my friend rebore aTriumph 650 with Nural pistons and Goetze rings but it was not so easy to find, some Triumph dealers sell New Hepolyte made in Taiwan but they suggest to replace rings with Goetze to avoid problems and they have in stock, we hope that will be also possible for our bike

Maurizio
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robcurrie
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Re: lot of smoke after rebuild, Help Please

Post by robcurrie »

Maurizio, unfortunately I didn't record the measurement of the rings, but I did measure the pistons. The groove depth for all the grooves is 3.58 mm and it was sufficient for the rings to have clearance.
(In a separate incident, a member was having problems with his 500 single engine heat seizing, turned out there was insufficient groove depth for the rings he was using.)
Hope this helps you.

Rob C
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1608
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Re: lot of smoke after rebuild, Help Please

Post by 1608 »

I recently bought a pair of Taiwanese made pistons for an Ariel 650 ( also advertised a suitable for BSA A10. The pistons appeared fine and have,so far, proved alright. But, I did replace the rings as the rings that came with the pistons were crap. The rings were cast iron and had a rough finish to them and measured 20thou gap in the new re-bore. I asked about and found it common practice that the rings are nearly always replaced with superior ones when purchasing said T'ese made pistons. So I never even bothered fitting the rings that came with the pistons.
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