Machining an aluminium cylinder

I have started to build the Karl Bengs hit and miss engine. It requires an aluminium cylinder to be machined with an internal step from 33mm to 35mm along with external fins. The supplied metal is almost to external 60mm dia. My plan: hold in 3 jaw Chuck and drill and bore to 33mm all way through, (my Chuck allows for bore upto 35mm). Machine a mandrel to 33mm and fit cylinder to it allowing for depth required for 35mm internal machining. Mount mandrel in Chuck and machine 35mm step approx half way through. Remove from Chuck and machine a 35mm mandrel that will take a running centre. Remount 33mm mandrel in Chuck and running centre on 35mm mandrel and machine fins.

is this the right approach?

Is the larger dia bore at the crank or head end?

If at crank end then I think I would hold in chuck, drill then bore to 33mm right through, bore the larger 35mm to length using a pre made ctr'd plug to gauge diameter. Then fit the plug and machine as much of the outside as possible particularly the spigot that fits the base so all is true and concentric.

Then remachine plug down to 33mm so cylinder can be held the other way round with Tailstock support to complete the fins at the head end of the cylinder

Jason once again thanks for such a quick response. By spigot I presume you mean the initial bit of the cylinder without fins that fits into the cylinder support of the engine frame. Your way certainly reduces the amount of machining of mandrels many thanks.

Yes, though looking on Beng's site I'm not sure now if there is one or if the cylinder liner is longer than the cylinder and therefore forms the spigot though same method can be used.

Bengs build

Similar sort of job on my Chuky but did not need the plug

From the plans the cylinder liner is 81mm long and the cylinder is 76mm. That would suggest the liner fits into the cylinder holder as they call it. It appears 0.5mm of 33mm dia liner protrudes and 4.5mm of liner protrudes at other end dia 35mm which I hadn’t picked up until now.

Edited By Andy_C on 04/08/2021 20:43:53