Piston Scoring

Lack of servicing.

How so? It get's its oil changed every 3000 miles, ignition and tappets about then too, carbs balanced every 500-1000.

The message from "sweller" contains these words:

They don't look too bad. How deep was it? Could you catch a fingernail on it, or was the scuffing too fine?

If I drag my fingernail on it I can just feel the scoring on the piston.

Have you owned this engine from new?

I've owned it for 13 years and those pistons have only been in there for two years and they weren't scored when they went in.

The message from "sweller" contains these words:

I've had worse with no ill effects. What's the compression and oil use like?

Compression is good - oil not bad but I suspect the valve guides are worn.

It's very unusual to get scoring on the gudgeon pin axis of the piston but almost none on the thrust faces. The pistons should be about 0.5mm smaller in diameter there than across the skirts. I'd say either overheating or incorrectly designed pistons - maybe a cheap aftermarket copy of the OE piston without the necessary ovality?

Looks to me like a partial seizure - lack of oil, and overheating can both be a cause, more likely to be lack of oil, as the cam followers are rather too worn for normal wear. I've had exactly the same in a twin, whereby the oil line to the head was blocked, unbeknown to me, and it just died slowly, like it had run out of petrol, it was very tight when kicked, so I left it 10 minutes, and it restarted fine to get me home, and showed the same sort of scoring when I dismantled it.

Possible low oil pump pressure? Maybe a partial blockage of the oil line to the cylinder head? Did you use gasket sealant around the o ring on the head/barrel interface? That is a usual place for oil to start leaking, so people put a small bead of sealer around the o ring, but this sealant then gets squeezed into the oil line. I'll give you £50 for it as is. Alan.

It was a Spanish police bike and by my estimates it has 250,000 miles on that cam and cam followers (assuming Plod didn't change them)

It's not - to my knowledge - seized. Those piston have only been in two years.

I last checked the pressures a couple of years ago and they were very good. I'll examine the pump for wear.

No.

I didn't use any sealer and it doesn't leak.

Would you like me to deliver?

From the pictures, the engine appears to have been assembled by a bodger, so anything is possible. There appears to be a severe lack of lube, overheating (possibly mistimed?) maybe not cleaned properly, not run in correctly, etc. Did the reborer know the correct clearances needed? Is the exhaust system the right one?

There are so many possibles, get it rebored, completely strip it and reassembled by a mechanic that knows the engine, obviously changing the camshaft etc.

Mrcheerful

What makes you think it's been bodged? What gives you that indication from the pictures?

What makes you think there's been a severe lack of lube - are there other clues other than the scoring?

No, the exhaust isn't original but it's jetted correctly (albeit slightly rich on the primary jets).

Timing is electronic and gets checked when I balance the carbs (it's a big twin so this is more or less every 1000 miles) it's always correct.

It's not been rebored they're STD pistons.

Problem with reboring is O/S pistons are more or less unobtainable.

Personally I think the pistons are recoverable but I'd like to know what the possible causes could be so I know what to look for when I strip it.

The bodging is indicated by the incorrect circlip positioning, in addition (although they might have been moved after dismantling is that the rings are incorrectly positioned (gaps)

Early V 750 M-G engines used chromed bores (not reborable at all) I do not know about the 850, but there was a problem with incorrect ring types being supplied (not compatible with the bore type) on the 750's. Chrome bores have steel rings, steel bores have chrome rings (IIRC from 30 years ago)

When the pistons were fitted, were the bores checked as the correct size for the new piston? I.e what clearance was available? What ring gaps as installed? Main jet sizing should have been checked and adjusted on a rolling road.

The cam followers show the lack of lubrication, the pitting is not consistent with wear, but is very consistent with poor lube.

In your case I would clean the pistons up, if the bores are steel then give them a light hone, replace the worn followers, reassemble and get rid of it, or use it.

There are really too many possibles on an old engine like this to know what has happened and when, but overheating (main jets?) poor lube and or dirt look like the main culprits, coupled with old age.

Mrcheerful

The message from "sweller" contains these words:

Well, that's probably where the oil's going!

There are no pegs for the ring gaps and the lining up is due to my handling between pictures.

Your point on the circlip positioning interests me as the manuals give no indication as to how they should be fitted and you're the first person I've ever heard who has said there is a correct and incorrect positioning. The gudgeon pin clips which on these are a sort of 'e' shape - even if they're incorrectly positioned I don't see it as evidence of a bodger.

This isn't a 750 it's an 850. Although the 750s had chrome Early Le Mans engines had iron bores, later ones and later T3s had Nikasil (a chrome coating). This has the correct rings for the bores.

These are matched bores/pistons both 'A' grade and off the same engine.

A long process of plug chops (I still haven't got my fingerprints back) and a lot of factory data determined the jet sizes. These have not been a problem for the past two years.

The followers have a reputation for pitting on these machines but your point on the lubrication failure is interesting.

Why get rid? I do use it - I've had it 13 years.

I don't suspect the jets as lean mixture tends to take out the piston crowns first (certainly in my two stroke experience) and lube failures damage the bottom ends. Which seems pretty unscathed. Which isn't that helpful in finger pointing.

Some further investigation into the oil pump and cam to see how that's damaged.

Overheating and they are picking up on the bores very slightly, or poorly made / machined / pistons.

Tim..

there was a spate of failures on some high revving Japanese makes, the failure was eventually found to be caused by the cross piece on the circlips, if sideways on to the up/down movement the cross piece tries to stay still and pings the circlip out. So, if you have the type of circlip with a cross piece, it always goes up/down, in line with the bore. some makers went away from circlips altogether because of this, most just did away with the cross piece, pattern pistons usually came with the e shape circlips, I used to remove them and replace with genuine circlips before selling the pattern kits.

Mrcheerful

Cheers.