1963 G12 Left Piston Seizure

gsneyd · Post by **gsneyd** » Sun Jun 12, 2005 4:26 pm

2000 miles of careful riding I opened my1963 G12 up on the local bypass to 70 mph. On shutting off the throttle the engine cut out and I coasted to a stop. The engine was restarted after a short while but had now developed an audible knock. Result on inspection. The rear of the left piston (only) had caught, a band about 2 inches wide was badly scored and the bore was marked with a similarly wide rough spot. I need advice on:
1. Why it could happen.
2. What I need to do now.
3. What I need to do to prevent it happening again.
Hope you can advise, as it looks expensive! [:(][:(][:(]

Biscuit · Post by **Biscuit** » Mon Jun 13, 2005 1:00 am

Graham, what oil do you use, and what type of oil filter?

Charles · Post by **Charles** » Mon Jun 13, 2005 10:18 am

Graham,

Assuming nothing wrong with the oiling system, this should require a new piston and rings and a good hone of the bore only. I had the same on my model 20 on return from Denmark and that is exactly what had happened to me.

Alan, I run mine on SAE 50 changed every 1000 to 1500 miles.

Debbie Green · Post by **Debbie Green** » Mon Jun 13, 2005 3:22 pm

Graham, When a piston siezes in a cylender the marks, there positioning and there color read like a book. lf you have a good small engine reco shop somewhere close they should be able to tell you why. lf not you can e mail me some pics and a bit more info l should be able to read it for you. (thats what l do for a living)

lawrence · Post by **lawrence** » Tue Jun 14, 2005 12:52 am

Hi Graham
Sorry to hear this happened to you. It happened to me as well. When the cylinder was rebored was it done by an engineer familiar with current practice on AMC's horrible cylinders and pistons? The new pistons expand more than the old and so the bore has to be a little bit larger than the old figures. I believe the figure is an extra one and a half thou but can check if you need to know. Also, was the weather very cold? Mine seized as it was five degrees below zero. I naively thought it would keep the engine cool and stop it seizing. in fact the reverse is true as it the same effect as removing a steel bearing from an alloy case but of course the other way round. I.e. the iron cylinder is very cold because of the airflow and the piston is very hot so expands and seizes. [:(]

Like Charles says it is easily sorted. Good luck with it[:)]

Lawrence [}:)]

gsneyd · Post by **gsneyd** » Tue Jun 14, 2005 10:36 pm

All. Thanks for your prompt responses. I will answer in turn.
Alan. Since the rebuild I initially used monograde 50's oil. This was changed after 100 miles. At 500 miles oil changed again, this time used modern 15-40's. After 1200 miles changed to Duckhams 20-50's Classic. I've always used the new filters from Jampot spares, the open gauze and felt type. At the last oil change the oil did not run from the oil tank (it had been standing over winter), so I prodded with a long plastic needle, I also scraped it along the bottom of the oil tank seam and I found some compacted debris. The oil tank was removed and thoroughly cleaned. I have never found debris on the main oil filter.

Charles. I am worried about what has happened to the metal removed when the pistons seized. I intend to do a complete strip and inspect all bearing surfaces have the oil ways blasted with compressed air, including the removal and replacement of the metering jets to give better access during the blasting process. Any comment on this operation?

Debbie. I'm afraid I do not have the technology to send pictures, but I will describe the marks as best I can.
There are two marks. A smallish rub mark at about 10:00 o'clock (the front being 12:00 o'clock). It was mid way between the oil ring and bottom of the piston, about 1" long and 0.3" wide. The second mark is much larger being like black rain between the oil ring and bottom of the piston and extended round the piston from 4:00 to 8:00 o'clock (about 2" wide). There is also a small depression in the cylinder wall to match.

Lawence. The engineering was done by a local engineer who deals only with motorcycles so should have been OK. The seizure happened three weeks ago at about 10.25 p.m. Cool but not cold. I have done 2000 miles since the work was done so I am of the opinion that everything was OK I have some concerns re the debris in the oil tank. Maybe this was the cause, and obviously had been missed at the original restoration, but it was compacted Hence my plan of action re my response to Charles.

itma · Post by **itma** » Tue Jun 14, 2005 11:15 pm

sorry Graham, it`s the multigrade oil thats done it, it just don`t do our sort of engines any good at all. I know there will always be a contra opinion on this subject, but I`ve seen so much of this sort of damage on these oils, norton and triumph main bearings used to fail rapidly on the stuff It`s a bete noir of mine, stick to monograde and a longer running in period, you won`t go far wrong.
I as for the bits of alloy of your piston, most of it will be stuck on the cylinder bore, so unless you are mega cautious a strip may be unnecessary

Biscuit · Post by **Biscuit** » Tue Jun 14, 2005 11:21 pm

Hi Graham, I almost said 'I bet you used a multigrade', but I didn't want to be WRONG again. I suggest you do a search of all forums, typing in 'Multigrade oils'. I did this this when I was considering changing to multigrades, purely on account of availability. After reading others opinions I stuck with mono's. Particularly interesting are the remarks on felt filters.

Adrian Pancaldi · Post by **Adrian Pancaldi** » Wed Jun 15, 2005 12:22 am

Hi Graham, I once had the same sort of thing happen to me. It was a coolish night and I was about 50 miles into a 250 mile trip. I backed off from my constant 65-70 mph speed to have a minor sieze. After letting the bike cool I kept going as I was the middle of nowhere. I was using a monograde 50 oil, though the jury is still out on that one for me. I would be looking at the piston (what brand?) to bore clearance as Lawrence says.
Regards
Adrian

Eric · Post by **Eric** » Wed Jun 15, 2005 12:54 am

Graham dont feel guilty if you were using multigrade oils, this in itself almost certainly did not cause your siezure. It seems that modern alloys require more allowance for for piston expansion than was used in the past as described by Lawrence elsewhere. Another member of our section whoes engineering knowledge and practice would be considered to be excellant, has been caught in the same trap more than once until he found out that you need the bore to be that bit larger than would have been used in the past. Multigrade oils must be used in the long term with an effective filter, the standard AMC units are not effective, but with a clean engine to start with and very regular oil changes e.g. 500 to 1000 miles they would not cause your problem. I have written extensive notes on this in the past but its all to do with the multigrade keeping debris in suspension and relying on it to be trapped in an effective filter. Non detergent monogrades will drop the the debris into all of the nooks and crannies of your engine where they will stay without causing harm unless distirubred, hence all the rubbish in your oil tank, but you cant clean an oil tank unless you cut the back out. Beware however monograde oils can have detergent in them that work in the same way as multigrades so its not as simple as mono or multi.