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[jawa-enfield]

1 The oversized are in plus 20 or plus 40.
Does that mean std. bore plu 0,20 or 0,40 mm?

2How much play should the piston habe, to what size should the barrel be bored out?

3 Finally, how do i recognise how the piston should be installed (which side to the front?)

That's all for now..

[trophyvase]

No question is stupid if you need to know the answer - unless you already know the answer!

The oversize is in thous of an inch - i.e. +20 = 20 thou larger than standard.

The piston to bore clearance depends on the piston to be fitted. These statistics should be info supplied with the new piston.

When the orientation of the piston matters it is usually indicated by one or other of these features

• Pistons marked ‘F’ must be fitted with the marked side of the piston to the front

• Pistons with an arrow on the piston crown must be fitted with the arrow pointing forward

• Pistons with cut-outs for valves must be fitted with the smaller cut-out to the front

• Pistons with a split skirt must be fitted with the split to the front.

• Where none of the above apply the piston may be fitted either way.

[PeteF]

You should supply the piston and any info that comes with it to your rebore man.

[jawa-enfield]

Ah, so the dimension of the piston is given in mm's, and oversize in 1/1000 of an inch.. logic..
Thanks for replie!

[Cranky]

All pistons have a major thrust side and a minor thrust side. Some have bigger skirts on the thrust side. Pretty important stuff beside any valve recess etc. Some gudgeon pins are off set too to balance thrust. This means a cut out on the skirt will be for the rod.

Yes the bore man will need the piston. imperative.

[jawa-enfield]

So, bore plus 20/1000 of an inch is.. eh 0,508mm
The bore original is - close to 84,0mm
I measure (a few times both sides and 90 degrees opposite)as accurate as my mutitoyo thingy (what's the name in Englisch?
84,0mm..

So, that completes the image since it still had the original India-tyres on it, and nuts/bolts on the engine seemed very virginal, it is the original bore..

Still, i am wondered (and worried) that in the few mileage (i estimate about 6000km) the cylinder has quite severe marks of wear..

Do some (inland) enfield have problems with 'soft'cylinder walls?

[Cranky]

Not sure what your problem is or what you are asking. Would you know what severe wear looks like.

No--cast iron sleeves do not have soft spots problems.

[papasmurf]

No--cast iron sleeves do not have soft spots problems.

The shouldn't, but it does not guarantee they don't.

[stinkwheel]

No soft spots but it is not unusual for the crown of a standard piston to collapse, gripping the rings in their lands then partially seizing the ring in the bore which can damage the cylinder walls or cause a huge amount of carbon build-up on the piston crown. Or even for the piston itself to come apart. Not unusual to replace the piston by itself and simply hone the bore in that situation if the bore isn't too badly damaged.

To assess bore diameter, unless you have a specific tool for it, the usual way is to slide a brand new plain piston ring down into the bore using the piston so it is sitting perfectly square in the "working" part of the bore then measuring the end-gap with a set of feeler guages. Compare the end gap to the standard listed in the workshop manual. If it's too large, you need to do a rebore.

A vernier type tool will not tell you about the diameter of the working part of the bore because it can only measure the diameter of the unused part at either end which does not wear. You would need a bore guage to actually measure it properly. (mutitoyo make both verniers and bore guages but I suspect you are using a vernier)

A quick and dirty way to assess bore wear is to see if you can feel a step in the transition from the working part of the bore to the unused part with a fingernail.

[jawa-enfield]

That's, litterally translated from my language a 'bumping edge'

the cylinder has no really wearing out but quite some scratches and at the lower half signs of severe wear.. maybe it had a piston jam some time