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I,ve bought a puller from minispares but neither of the 2 threads seem to fit in the flywheel. it's a verto clutch so I assumed it's a m10 bolt but that's far too coarse. any advice before I damage anything.

Before giving up on imperial sizes, try 3/8' UNF (3/8-24). That's what the older pre-verto flywheels are. I would be very surprised if the fasteners were metric.

the engine's out of a 1996 mpi.

Yes, the Verto flywheels did cross over to metric. Steve (CTR)

CooperTune wrote:Yes, the Verto flywheels did cross over to metric. Steve (CTR)

I learn something new every day. What size and pitch are the metric taps?

M10 x 100mm grade 8+ bolts for Verto

MAPP or oxy torch and a lump hammer are also useful for more obstinate flywheels...

the m10 bolt is far too large and the thread's too coarse. the unf bolt looks the correct size but just doesn,t seem happy to thread in, if you know what I mean. can someone have fitted an earlier clutch or is that impossible. I haven't worked on verto clutch before.

its a long time since I had to do this but bearing in mind my day job involves removing some pretty heavy duty separating here are my thoughts.
I have an ancient puller 3/4 thick , but took one look and thought no! At best the centre bolt was 14 mm metric. So looked in my tap draw and the largest I had was 7/8 unf 22mm so I opened the puller up to 7/8 unf That is about the largest bolt you will find kicking about with a thread pitch under 2mm though m20 would be a good second best.
I then ordered new 3/8 unf bolts 4.5" standard are 4" but I made 13mm en24 thick washers as the puller was slotted. BUT you need good bolts so I ordered sae grade 8s which are 10.2 kn not 8.8. I didnt choose cap heads as unless you can get unbrako (ARP) you may get common often brittle Chinese ones.
So put it all together tightened it up with my 1" drive 1&5/16 socket and 2ft 6" proto t bar. Nothing its my old mk 1 s and been together a long time. Hit the 7/8 bolt with a 14lb hammer still nothing.
Went home thinking next morning I will have a Mark miracle, unfortunately not, contemplated Wurth Roost off ice, kettle of hot water, gas axe.
Sod it picked it up took it to big compressor. In the days when we ran a lorry we used a 1" drive air wrench. Was worried about the 7/8 thread as the puller is basically mildish steel. So turned the air gun right down. Rattled a bit nothing, turned it up a notch rattle rattle tightened the bolt then kazam off it came.
There is no doubt that with a good puller the air wrench vibration and torque is the way to go . Perhaps not an inch drive unless you have confidence in the puller.

I have a home made puller made in the 60,s .. i wind on as much safe torque to apply on the Puller..then with a soft Copper Drift give a heavy wack onto the back side of Flywheel through the starter Hole & this has always worked for me.. However take care what type of material is used as your Clout Drift so as the starter ring is not damaged.

In general, automotive metric fasteners are finer pitched than standard B&Q / Screwfix offerings.