Piston with odd marks on top.

Per · Oct 24, 2021

Hi.

When I had the cylinder head off, some odd scratches or marks became visible on the right hand piston. Does anyone have an idea what this could be due to? Se picture.

Regards

Per

mean gene · Oct 24, 2021

If your talking about discolored area, my guess is that a valve was open and the carbon collected moisture and began to etch piston. Engine oils have sulfur in them H2O combined with sulfur create sulfuric acid!

Per · Oct 24, 2021

Yes, I'm talking about the discolored area. Your guess sounds reasonable. The bike has been stored for almost 20 years.

Triton Thrasher · Oct 25, 2021

Agree looks like oldness!

concours · Oct 25, 2021

Corrosion.

EstuaryBoy · Oct 25, 2021

Concours concurs... correctly corroborates corrosion!

(Sorry, long afternoon ahead....)

Seeley920 · Oct 25, 2021

Now tell us about the rest of it.... big valves, skimmed head for a start

kommando · Oct 25, 2021

Besleeved barrel ? RGM 920 kit possibly.

Per · Oct 28, 2021

920 kit bought from Fair Spares in 1982.

concours · Oct 29, 2021

Why would someone stamp "STD" on a big bore set?

Fast Eddie · Oct 29, 2021

concours said:
Why would someone stamp "STD" on a big bore set?

They’re proprietary car pistons from the day.

EstuaryBoy · Oct 29, 2021

Norvil list two oversizes for their '920' pistons...

robs ss · Oct 29, 2021

concours said:
Why would someone stamp "STD" on a big bore set?

To differentiate it from 20 & 40 thou oversize?

kommando · Oct 29, 2021

Per said:
920 kit bought from Fair Spares in 1982.

Heavier pistons than the 850 in the Norvil 920, not helped by bigger pins, so crank needs rebalancing, RGM 920 std bore pistons weight same as 850 std OEM pistons and pins are same size.

Per · Oct 29, 2021

kommando said:
Heavier pistons than the 850 in the Norvil 920, not helped by bigger pins, so crank needs rebalancing, RGM 920 std bore pistons weight same as 850 std OEM pistons and pins are same size.

Yes, I had to rebalance the crank and also make the small end bigger. I'm not sure if the 920 kit was available from RGM in 1982.

Per · Oct 29, 2021

Seeley920 said:
Now tell us about the rest of it.... big valves, skimmed head for a start

It's not much to tell. Except from 920 cc, gas flowed head and some lightenings here and there the engine is quite standard. I did this 40 years ago.

Fast Eddie · Oct 30, 2021

Top man.

And folk say 920s don’t last !

EstuaryBoy · Oct 30, 2021

Fast Eddie said:
Top man.

And folk say 920s don’t last !

And that Norvil only sell cr*p!

Piston with odd marks on top.

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