Clearance between piston and cylinder barrel

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Sep 28, 2004
I have just bought a 850 commando, there was a little oil leaking from the
head/cylinder gasket so I disassembled it, now that it is apart I wanted to check the piston, rings and cylinder barrel.

If I meassure the int. dia. of the cylinder and the ext. of the piston and extract the to readings what schould this be as a max.
I have around 0,25-0,3mm.
The barrel measures app. 77,00mm what to me nearly looks like it is not worn at all.
How do I check the rings, I tried to put them in the barrel and measure the gap with a gauge feeler it is app. 0,4mm

Best regards

Your Lucky, The Cylinder block at 77mm (3.0315) should just need a honning so you will scuff in a new set of rings.
The Pistons are history because you need .1016mm (.004) clearance not .3mm (.0118).
Rings are history because you removed them and thier at the max anyway. Yours at .4mm(.0157)Ideal is .2mm-.3mm (.008-.012)
So a new set of .254mm (.010) over size Pistons and rings and a agressive hone job should set you straight.
That's if you can find a (.010) over set (.020) is more common. Don't worry if that's all you can find it might give you the opportunity to straighten out the bores that are often found to be not so straight after all.
About 6 months ago Phil Reed at Faire Spares had a set of 850 .10 over rings.
As I understand a set of oversized rings wont do it if the gap between the piston and the cylinder is to big (0,27mm total (bore - outside piston)) this schould give a gap of 0,135mm on each side of the piston.
Does any body have the measurements of the cylinder and piston with tolerances from new.
Luckey, In the Workshop manual Taking the largest bore allowed and the smallest piston You would have (.005) clearance or (.0025) per side clearance. If you have (.005) per side the bike would run with rings fitted but you would have piston slap.
Pistons are only measured at the bottom of the skirt 90 degree to the wrist pin. If you are looking at the piston anywhere but here you will find much smaller measurements.
Your clamed 77mm bore is near perfect stock size. But will require a hone job so you can wear in a new set of rings.
If you recheck your pistons and find they are less worn than your first set of measurements you may find that you have (.004) per side and I have seen lots of goood running Nortons like this as long as they have fresh rings. norbsa
Just a quick word about piston-to-cylinder clearance: pistons will expand with heat and different materials expand different amounts. So, recommended piston clearance will vary with the brand of piston.

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