Here's how I do it:
1. Cut almost through ring gear with angle grinder and cutting disc. DON'T nick the flywheel.
2. Split gear off with a hammer and chisel. Wipe flywheel surface where it sat. Sit flywheel on floor with clutch running face upwards.
3. Sit new ring gear on 3 bricks (firebricks best, otherwise use dry housebricks) with chamfered edge of teeth downwards.
4. Rub top surface of ring gear with emery cloth to polish it a bit.
5. Using an oxy torch, heat the gear evenly all round until it is a dark straw temper colour. Don't let it get to blue, it will soften it.
6. Using 2 pr of tongs or multigrips, pick ring gear up and drop it on the flywheel. Tap it gently and quickly if not fully seated.
Note- you will seldom get enough heat for this job with a Primus etc blowtorch, unless it's indoors and it's a warm day. Oxy is so much better as a source of heat. The last thing you want is to have it stick, 1/2 way on...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
