flywheel lapping

[JonE]

ok, blue bought! Got grinding paste. I'd glad Bob has mentioned about the relief in the middle, as I'd seen that and wondered whether someone had buggered about with it. Phew.
I'll research lapping pressure... thanks.

[JonE]

OK, dial gauges. Imperial.
A 1" one or a 1/4" one - what else would one need it for?
Is a new Chronos 14 quid one from ebay better than a 50s or 60s UK made secondhand from facebook marketplace?

[Colin Wilks]

Hi Jon
I'm not completely clear what you're measuring. I assume the run out of the flywheel, but are you doing this before or after fitting the crankshaft in the crankcase?
Either gauge (1" or 1/4") will measure to .001" and I don't think you need less than that. The total range you're measuring may well not exceed .020", and hopefully a lot less than that. Woodrow reckons run out should be less than .004" between extremes (i.e. total movement, not just deviation from the mid line) and if measured with the crank in situ you should address anything over .015", which seems a lot to me.
Personally, I don't begrudge spending on precision measuring devices, but in this case I'd buy a cheapie new 1" dti and put some thought into how to mount the crank and flywheel for measuring run out, and spend more on buying a decent stand for the dti. Cheap ones are a pain.
Another use for your new gauge could be to establish TDC by bracketing the piston travel, as has been discussed on here recently.

[Bob Culver]

Dial gauges are useful if you own a lthe but limited use otherwise. Sometning on a rigid surface moved against the run out surface will be pushed out by the wobble when rotated. The clearance can be assessed by eye or feeler gauge.

[JonE]

Colin - was suggested for runout, yes. But I like Bob's post too! (All extremely confusing but this could be a spare if the Woodie engine takes longer to attend to). The crank is currently completely immobile in the clean engine and the nuts on the journals I couldn't even open with a spanner once the splitpins were removed. But I suspect it may have once been diligently 'attended to'. I have no intention of taking the crank out if it all rotates fine once I've free'd it up.