G9 new rebore - seized piston

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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David1977
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G9 new rebore - seized piston

Post by David1977 »

Hello , I am in middle of engine overhaul as mention in previous thread. I did overhaul uper part of the engine 4 years ago. new psiton 0,4 rebore 67 and new valve and guides but When I removed head and cylinders I noticed that left piston is damaged . Piston shows more wear on inlet valve side. I thought it is not big deal but than I took closer look and I found out that scraper piston ring is a stuck in groove . I think I was litle lucky because the piston could have seized . I need to mention that I had the piston made here and I did not order them throuh club or AMC spares. So the question is what to do now. Our mechanic said that new pistons will be needed . I need to mention than engine was running OK without any problem.
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Groily
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Re: G9 new rebore - seized piston

Post by Groily »

Looks as if, perhaps, the rebore was little tight and there wasn't enough clearance on the skirt of the piston on that side. Can't really tell how deep the marks are from the pictures, but in whatever case, not good news.

If the bores are still undamaged at 67mm (which is +40 thou" in Brit-speak or +1mm in metric), then new pistons are available from the Club. If a rebore is required, +60 pistons are also available.
You say it was running well, and I assume not pouring out smoke or using lots of oil. (You don't say how many kilometres it had done in the past 4 years though.)

The brand generally available is called JP, which some people don't love much. I have them in my 650 though, and they have been OK. BUT - they do NEED a generous skirt clearance of I would say 0.0045" or 0.005" on a 67mm bore or there is the risk of trouble. So it would be no good just fitting them without having the bores checked first. (Sometimes, Hepolite or AE pistons come up for sale, even original-type wire-wound ones, but not as often as many of us would like!)

Maybe you could get the stuck oil control ring out and polish up that piston so it could serve for a bit longer, but I'd say it wasn't worth the effort and I'd follow the advice of the local engineering people. That way, you'll KNOW everything is good, as opposed to having that worried feeling that comes when we know something isn't really right.
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David1977
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Re: G9 new rebore - seized piston

Post by David1977 »

Hello Bill , you are right I will let check bores of both cylinders by local engineer and than I will get new pistons If needed . I will go to Staffod show in April so I will probabaly collect them with other parts but it little bothers me because I have done only 4000 km since new pistons and rebore .
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Les Howard
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Re: G9 new rebore - seized piston

Post by Les Howard »

I would (not saying you should)1) Remove the pistons. 2) Heat the piston crown up with a blow torch (don't go mad) this with some wiggling will release the stuck ring/s 3) With a very smooth file, clean up the scoring and finish with 600 -800 grit wet and dry. 4) Measure the piston skirt and piston bores and hone to get 4-5 thou clearance.5) Rebuild as normally......... Now this is a little extra point of view. I'm not sure what oil you are using? I reckon some modern oils are too good to use for running in an engine. The bore of an engine needs to wear and bed in, but imagine that it lubricates to such a level of perfection that no wear can take place...so even after hundreds of miles, this will leave the piston still with an as new, raw fit and the slightest bit of overheating will have it expand and seize. When running-in, those small contact areas are removed by normal friction if the oil will allow a tiny bit of wear so they bed in with sufficient clearance when very hot. So maybe a "running-in" type of oil might suit your engine for say the first 500 miles and then a conventional multigrade, but not a synthetic type. Also don't use too heavy a viscosity oil as there will not be enough oil being flung from the big-ends as they whirl round. Too heavy an oil will achieve a high oil pressure and then simply blow off the surplus through the oil pressure relief valve thus short circuiting the full flow round the engine and out through the big ends...The more oil that is flung from the big ends the greater the cooling will be on the pistons. Remember the expansion will be linear vs temperature so just by keeping the pistons just a tad cooler, perhaps even say 20-50 degrees, it might just be enough to stop them expanding to make high pressure contact with the cylinder walls.
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