V50 Monza cyclinders

Whilst yet to be dismantled and measured, I suspect my Monza requires a rebore but wonder if the bores are Nikasil coated. From the workshop manual specifying three piston sizes it appears not.

Any information will be greatly appreciated.

I think that you will find that they are. The three pistons sizes are not for oversize. They just get matched to the bores to allow for minor machining discrepancies.

What makes you think a rebore would be required? I pulled my V50 down at around 55k miles and the bores were still like new. I just needed new rings.

They will be coated, Nikasil. If they need repair and replating this co comes recommend.
Like cyclobutch never had an wear problems. Pistons maybe, usually rings.

Compression is down to 9.8 psi on one cylinder, so I thought the worse.

I will check the pistons and replace rings when my Aladdin stove can sufficiently warm the garage.

Very many thanks for the advice.

I think a compression reading as low as 10psi is indicating a major problem and if you haven’t got a holed piston, my money would be on the valves. Even with broken piston rings you usually see 50 or more psi. For the compression to be this low, I would be looking for a bent, broken or badly burned valve. Another possibility would be a blown head gasket, but that’s usually pretty obvious!

Going back to the original question, I’m pretty new to Guzzis, but my reading indicates that early bikes, pre-1982? - had chrome plated iron or steel liners in the aluminium barrel - this is what my 1980 V50 II appears to have. I believe Nikasil was introduced in the early '80s but I would appreciate it if someone who knows about this bit of history could confirm or deny! I have also been told that there could be a tendency for the chrome plating to flake off in fragments on some bores, thankfully mine were OK. If the bores have any significant wear, you should be able to feel it with a finger at the bottom or top of the swept area - don’t be misled by the ring of carbon deposited at the top of the bore, above the top ring. Both chrome and Nikasil are extremely hard wearing.

Both chrome and Nikasil bores are problematic for re-boring and it is not normally done. They can sometimes be re-plated if your wallet is big enough (it won’t be afterwards!).

Once again, someone please correct me if I’m wrong about the use of chrome and Nikasil on V50 bores, I would like to know the true story!


That sounds low. What year is the bike?

The changes occurred about that date. Mine is 1984, but I know it’s had previous engine changes. The engine now came from a Monza courtesy of Gutsibits. The Monza and the mk3 has the same engine. It has Nikasil bores. There are always horror stories because mechanical failures happen. Your chrome is fine, as are many others. Nothing is cheap now as the parts become unattainable, but most parts can be repaired.
I’m now searching for pistons and shells but it looks like I’m going to have to get the pistons made. That will be about £250 (each). A job for next winter. :open_mouth:
There are some good books to read, Mike Walker’s MG restoration, Guzzilogy etc.

I don’t think you’ll find a V50 on chrome bores. They will be Nikasil, or maybe Nigusil (same stuff I think, just MG branded version).

Cyclobutch is correct. The V50 was the first of the new engines with Nigusil bores. The bores don’t wear, pistons are pretty tough. Even with sloppy pistons you should get good compression. It’s all on the rings and their fit in the pistons. Get valves seats recut. Check lip of valve is not too thin. All dimensions in book. V important is good fit in valve guides particularly exhaust. It is the only way the valve can loose its heat is through the guide. Had to keep pulling the top end off my V50 which I bought second hand. One cylinder just wouldnt run right even though a compression test indicated compression well within range. I purchased a cheap leak down tester at machine mart. Basically putting compressed air into the cylinder and measuring the ‘leak down’ on a gauge. Listening for leakage through the oil filler, exhaust and inlet ports indicated leakage. Rings replaced even though they looked ok-end gap at max though. Could hear air escaping into ports even though I had lapped valves. So replaced rings, exhaust valves, guides and got valves cut as per book. Runs perfectly now.

Just looked at my compression gauge with a magnifying glass. I should have written just under 10 kg/cm2 or 10 bar. The psi figure is 140.

I followed a tip to pour a little oil down into the bore and retest. This increased the compression to 12 bar or 170+ psi, I am told proving the problem to be rings rather than valves.

Having had the carbs sonic cleaned, I will be throwing the kitchen sink at the engine: new valves, bronze guides and seats as well as piston rings.

Once more, many thanks for all the advice.

Garth Forbes


So now the original issue is resolved can i ask - does the above apply to the V35 as well? I have an Imola so am curious.

My guess is that the information would apply to the Imola as well but I am not an expert.

Garth Forbes