LCGB Forums

Help needed Oil seal has blown so I split engine and found the piston was looking a little dodgy. The first set of photos are from today. The next lot are from breaking down on the way to Austria last year. We thought the problem had arisen then because of the reed petal snapping. Philipe from BGM initially thought it was a problem with the piston but once the reed was found to be faulty he and others thought the reed had caused the problem to the piston. This time no faulty reed. Scooter has done approx 1500 miles since then. Varitronic ignition can't remember what timing is set too but was set up with correct timing. Another thing that came to light as I stripped it down is the bearing collar was loose on the crank and I was able to pull this off with my hand. Evo exhaust, Mikuni 35mm carb. This piston was an Asso, the one in Austria was a BGM/MBD piston. Any ideas?


Crown has 'burnt' away



Top of piston



These are photos from Austria's breakdown. The small indent was thought to have been caused by reed petal failure. Just had the scooter dynoed and not running weak. Crown has a bit 'burnt' away





I had hoped to be out and about this weekend for a trial run before Whitby. Oh well....
Any thoughts? Also will my crank be ok with new collar and bearing. Mystified why collar was loose.

Looking at both, I'd say leaking on exhaust manifold or exhaust blocked leading to a build up of heat. The MB piston looks to have got very hot possibly fuel starvation or crap petrol? The Asso looks very oily and a dark crown, maybe gear oil leaking which will cause overheating . Something I've come across is alloy cylinders bellying mid bore which may have caused the ring to catch the ex port and gradually pull the pin out. There can be many reasons for the damage , these are just my observations.

Had an almost identical one to the MB piston on a malossi 172.
It had done many trouble free miles and had rings changed with a 1000 before it happened. I couldn't find anything wrong when I stripped it put a new kit it as piston/cylinder shot and it has never skipped a beat since.
I could only put it down to bad fuel.......

Retarded ignition will cause the flame front to move towards the exhaust port increasing heat in that area, Anyone with an egt will have noticed that if they run a variable advance/retard system.

Some food for thought. Strangely this problem has only started since I changed exhausts a couple of years ago. The exhaust isn't leaking as far as I can tell. The seal has blown and that might explain the oily look of the Asso. I'm having the barrel checked by a good engineer and will check the timing because that may be part of the problem. I have now got to get the flange plate off. One ruddy screw well and truly stuck. Been toying with the idea of an egt and this may now push me towards buying one. Chuffing Lambrettas

Both pistons have the same peg damage so I'm wondering if the boost port or ex port have been widened? As you have said, get the engineer to check the bore is even all the way down.Presumably the crown is level with the ex port floor when at BDC?

Wack wrote:Both pistons have the same peg damage so I'm wondering if the boost port or ex port have been widened?

Good point, is there any sign of the rings snagging in the exhaust port? usually looks like the ring are trying to 'square' the top of the port. The flexing of the ring in and out of the port can loosen the rings pegs or so I have heard.

Just an update. Barrel k-nackered needs relining. Crank k-nackered. On fitting new bearing collar it just slipped on without any effort. New one on order. Barrel bore ok and ring pegs didn't snack on ports/transfers. Still not sure on cause but plenty of ideas to consider lol.

Won't some loctite bearing fit sort your crank problem?

Thought of that and have now tried it but worried it won't hold it permanently. Been given info about a local engineering firm that somehow build up the metal to hold the collar. Apparently this is very strong so will be paying company a visit to see what they can do.

nickw wrote:Help needed Another thing that came to light as I stripped it down is the bearing collar was loose on the crank and I was able to pull this off with my hand.

What crank is it? Had same thing with one I had fitted. Manufacturer changed crank under warranty. PM me if you prefer.

Your crank bearing journal must be undersize. Presuming that your NU2205 bearing was good quality (European, Japanese) it should be re-useable.

I’ve experienced the same problem.

The way I fixed it was to measure the difference & was able to select a Loctite product that would cope with the gap. I made sure that the surfaces were scrupulously clean but also cut a groove by hand (junior hacksaw & diamond files) for a snap ring. They are like a circlip without ears.

It was a bit ‘belt & braces’ but worked well.

I’ve also seen the journal knurled up to raise the surface (like a miniature bed of nails) & that worked too.

It all depends what you have access to, but years ago, even crankshafts were not so easy to come by!

Martin it was a BGM crank 60mm 110 rod. Put in after the Avignon euro 2014. Apart from collar crank is fine and not out of true. Nightmare trying to get a new crank - none in stock. Hopefully have now got one sorted. Wanted to get the engine ready for Whitby but have now just about ran out of time due to work commitments.

Warkton Tornado No.1 (just love that name) I've replaced the bearing now and it is the new collar that slips on and off. Both old and new bearings are SKF and were European. I have now Loctited it on and it does appear to be very strongly held. I have read about people dot punching around the mating surface to get a better grip but didn't want to do that myself without further investigation. I like the idea of using a snap ring but would leave that to someone else to do. I can't sharpen a pencil without breaking the lead. I have heard of a local company who say they can 'spray metal' the surface and then hone it back to ensure the correct fitting of the collar. I'm off to see them when I get the chance.
Why would the first collar have come off? Would it be due to excessive heat and would the collar spinning wear the crank face?
cheers for the input

If done correctly, that sleeve should be good to run as Loctite commodities are serious engineering products.

If I were you, I would run it & not be tempted to disturb it further by centre dotting or anything like that. Just keep an eye on it…..

Metal spraying is also a good process but I would keep that idea ‘in hand’ if the Loctite fails. I’ve used the process years ago when I’ve over-ported & broke through the base of a DT175 pistoned barrel.

As for the reason of the journal being undersize, I think it’s just down to poor quality by the manufacturer, not from slipping in the first place, although that will have made the issue worse.