The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: old PJ on 14.12. 2008 10:01

Title: Piston melt down
Post by: old PJ on 14.12. 2008 10:01
Help . I have rebuilth my engine using all new parts and had all the engineering done to spec and timing spot on advance retard working ok correct spark plugs new carb . So why is the oil pump side piston  getting very hot and melting . I have taken the engine apart and all is well at the bottom end of the engine loads of oil getting to the parts that need it its just the top of the piston which is getting hot its like a welding rod has been going at it and it melts and traps the rings this is after 1,200 miles so it takes it time before it gives up . This has  been the second rebuild . And the piston which is on the clutch side of the bike is fine no problem at all and thats the side which use to be the problem with oil getting to it and the con rod had the mod with the oil hole.

Old PJ
Title: Re: Piston melt down
Post by: Brian on 14.12. 2008 10:31
This doesnt look like a oil problem, more a combustion problem. Lack of oil would cause the piston to sieze in the bore. Does the engine ping when under load? Like a rattling sound. If it does then there can be a few causes. The most common being poor quality fuel. Another possibility could be a worn cam ring in the maggy. If it was running too lean then I would expect it to hole the piston rather than eat it away as it appears to have done.

As it has done it twice it would pay to get some help from someone near you who has some experience before you ride it again.

This is by no means a common problem so something is definitely wrong.

Title: Re: Piston melt down
Post by: LJ. on 14.12. 2008 10:44
Another possibility could be a worn cam ring in the maggy

You may well be correct there Brian. I know the bike the engine came out of, I think there are pictures on the forum here of a holed piston from the last time it blew although that blow was on the drive side, maybe the bike was timed with the leads the 'other' way round and now attempting to blow the oil pump side piston? Good thinking!

My question... Could an Induction bias carb cause such damage to piston tops??
Title: Re: Piston melt down
Post by: groily on 14.12. 2008 10:54
Nasty one PJ. Agree with brian, not an oil problem - an ignition/valve/fuel one. Is it the exhaust valve side that's been eaten right through to the rings? Looks like terrible detonation, but I'm sure it would have been rattling away like anything, as Brian says, to have done that much damage - twice!. Assume the plugs are a) both the same and b) correct for the engine. What do they look like I wonder?
With those pistons, octane rating shouldn't be a real problem I wouldn't have thought unless your fuel supply is adulterated. Number 1 thought is also (a quite large!) timing variation between cylinders, perhaps compounded by weak mixture (induction bias?) and less than perfect valve or seat. What do the underside of the head and the valve heads look like now? Are there great big lumps of hard-as-concrete stuff attached in various places causing pre-ignition through being incandescent all the time? And the inlet and exhaust tracts in the head and the backs of the valve heads? Are they all reasonably clean?
Whatever, it's not good or pretty, but it'll be one of these things or a combination - best of luck.
Title: Re: Piston melt down
Post by: a10gf on 14.12. 2008 11:01
Definitely check the magneto L\R accuracy as a 1st step.
Title: Re: Piston melt down
Post by: old PJ on 14.12. 2008 18:45
Hi all The head is in very good conditon had all new valve seats and guides and valves and all seated in nice . So it must be the mag or carb that is the problem . The piston got very hot in the bore and it has a split down the skirt . The bore is ok no marks and i have check the tolerence and its fine so a set of new pistons this week  and get it back on the road  soon.

Title: Re: Piston melt down
Post by: A10Boy on 14.12. 2008 20:02
PJ, Did you have the head work done before or since the piston failures? I'm thinking that you could have a loose guide which is sticking or bouncing a valve intermittently.

Weak mixtures normally cause holes not burning like that, I would change the mag for sure. Do you use an octane booster ?

When it runs, does it tick over smoothly or is it really lumpy, most cases of induction bias wont tick over sweetly at all.
Title: Re: Piston melt down
Post by: groily on 14.12. 2008 20:26
Sounds as if the engine has a bit of a history PJ. More and more I think you should check the ignition timing on both sides very carefully - and if necessary get the cam ring replaced - before riding the thing again with yet more new shiny pistons! Check all mag parts in fact - sloppy bearings can have a noticeable impact on timing via a hugely varying points gap. Induction bias alone is unlikely to have done all that damage but it could certainly have contributed if there are other things that are not quite as they should be. Normally, the rearmost mag HT pick-up goes to the right hand cylinder - but as you imply, it doesn't matter much in a parallel twin and they could have been swapped if someone timed it on the other cylinder. So could well be (likely even) the previous problem was caused by the same thing, although if it holed before, we might be back looking at the carburation as well. I'd also check carb flange to head joint for leaks (though that ought to affect both cylinders) and be very fussy with the valve clearances. I only ever had one piston hole itself - it pinged like hell for a few miles despite my trying to nurse it the rest of the way home, gave an exasperated sigh and went onto one. At night, in the rain. Of course.
It can only really be ignition timing or mixture or a combination of the 2 when something like this happens on one side only.
Title: Re: Piston melt down
Post by: old PJ on 14.12. 2008 20:29
All ok in the head valves new and guides are tight and its been getting hot all the time it must be the mag or carb will check it out soon.

Title: Re: Piston melt down
Post by: old PJ on 14.12. 2008 20:38
The bike will run ok on tick over and it rides ok the last time it melted the piston bad on one side i have done the timing again and it still got hot and started to melt the piston again it must be the mag i will check it out . Thanks PJ
Title: Re: Piston melt down
Post by: Brian on 14.12. 2008 21:11
Well PJ all that advice should give you some things to check which is what the forum is all about.

The reason I mentioned fuel quality is because where I live fuel quality has been a huge problem for all of us that ride old bikes or drive old cars. I use a 98 octane fuel and still get some slight pinging under extreme conditions. Most of use dont run quite as much advance as recommended. We have found if you run the bike slightly rich and with not quite as much advance they run fine. I have a friend with a Goldie and it simply wont run properly on pump fuel regardless of the octane. He uses a full leaded 100 octane aviation fuel. The trouble with modern fuels is regardless of the octane rating they dont have any anti knock qualities, modern cars with variable ignition and cam timing can cope but our oldies dont fare so well.

This may be a problem that lies with the fuel suppliers in my end of the world or may be more widespread. 

Title: Re: Piston melt down
Post by: old PJ on 15.12. 2008 18:32
Hi all well i have taken the new carb of and its all looking ok just the main jet is a 300 and the books say 250  so it looks like the magneto has a problem with the cam ring so i will take a look at that next .

Thanks for help .
Title: Re: Piston melt down
Post by: dpaddock on 15.12. 2008 19:24
This engine is a Flash with the iron head, yes? What compression ratio? What head gasket type - laminated or solid? What color(s) the spark plugs?
Title: Re: Piston melt down
Post by: old PJ on 31.12. 2008 12:21
Hi all i have been working on the engine over the holiday and just found out that the cam is for a road rocket  and not a standard A10 so would this make a difference to the timing i have standard flat top pistons and iron head thick flange barrels i have been using the timimg 11/32 BTDC fully advanced the book recommends road rocket to be set at 3/8 BTDC fully advanced as i say i have standard pistons on a standard engine set up so which do i use ?

Title: Re: Piston melt down
Post by: Richard on 31.12. 2008 17:16
I do not pretend to be an expert on cams and timing however I was told that the petrol today is different to yesteryear and the timing on my S/Rocket should be altered to 5/16"btdc fully advanced which is not far away from your 11/32"btdc.The thing to remember is that you must time fully advanced so if you have an auto advance pinion on the mag then fit a 6mm bolt in it to open the bobweights before fitting it then recheck the timing again in case anything moves. the setting of the points to the correct gap is also important before timing and check see that you have the same gap on both cams of the slip ring.
A good bet that the timing is good with the auto advance fitted is that you get no noticable kickback on starting with a crisp sound and sharp response to the throttle. if it sounds or feels flat recheck as it may be retarded.
This is how I do mine and I hope I am not teaching Grandma to suck eggs
All the best
Title: Re: Piston melt down
Post by: A10Boy on 01.01. 2009 18:50
Before you worry too much, I would suggest that you check the part numbers for the camshafts, as later A10's used the same cams as the super rockets so yours might not be wrong.