4 cylinder rotary valve

This topic has 8 replies, 4 voices, and was last updated 17 May 2026 at 14:02 by AStroud.

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5 April 2026 at 09:20 #843291
AStroud
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@astroud
Having had some success building engines with rotary valves, namely my single cylinder

https://youtu.be/gVItba_ImAM

and my Coomber engine

https://youtu.be/iVgsvn_kNdE

I thought I would try something a bit different. This will be a freelance 4 cylinder engine using an overhead rotary valve driven by a timing belt. Cylinders are 22mm copper tube, yes I know not the best material friction wise but should be OK for a model and the tube has been knocking about in my scrap tray for too long.

The crankshaft is already made, 0.75 stroke, so I am aiming for a relatively high speed engine. So far the build is proving to be a test of making / machining 4 identical items or dimensions, something I have not really needed to be concerned with when making single cylinders.

If there is interest I will post more progress reports.

Andrew
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5 April 2026 at 09:58 #843304
JasonB
Moderator
@jasonb
Always interested to see people actually making things.

Fid you skim the inside of the tubes or are you hoping the drawn diameter will be OK?
5 April 2026 at 13:50 #843353
AStroud
Participant
@astroud
The internal surface finish is quite ok, but as you say I might be found out on the consistency of the internal diameter, I will find that out when I make the pistons. Following your experience I make my models without piston rings but if I have a id issue then I might use a larger clearance and O rings. Failing that then I would make new cylinders out of brass or aluminum as I don’t fancy my chances of skimming the copper tube.

Andrew
7 April 2026 at 16:40 #843609
old mart
Participant
@oldmart
An expanding lap would be a way of truing up the bores while taking the minimum of metal out. I have recently seen an advert or article about lapping compound which is easy to remove, but cannot remember where.
11 April 2026 at 14:56 #844198
AStroud
Participant
@astroud
Some progress to show, I have made ties between the head and deck, supports and a centre bearing.

The crankshaft will be run on ball bearings and I next need to make a couple of bearing housings which will be fixed to each support. These will allow some alignment adjustment with the centre bearing. In truth I don’t think a centre bearing is needed but I think it would look a bit odd without. I have a separate shell for the bearing which is yet to be split and I hope it will also position the crankshaft axially.

The head is a bit blocky at the moment, may try some shaping later on.

Andrew
15 April 2026 at 00:52 #844513
Peter Hoerlein
Participant
@peterhoerlein37724
Thanks for the comment. You build some interesting engines. Nice work too.
15 April 2026 at 11:26 #844564
AStroud
Participant
@astroud
Thanks for the comment.

Bearing housings are made, next will be the connecting rods and pistons, you will notice I am leaving the challenging rotary valve to last !

There might be a brief hiatus in progress. What started off as having new smart meters installed has resulted in our gas supply being cut off, condemned as the service pressure is too low at c. 15mbar. The theory is the new plastic liner fitted 12 months ago in the pipe from the street to the meter has kinked or collapsed. We might get away with a new section of pipe, worst case is all new piping and a new meter position which means big disruption in my workshop where the meter is.

Andrew
4 May 2026 at 15:18 #846869
AStroud
Participant
@astroud
My gas supply all sorted apart from the remote display which refuses to show any data, annoying as it was the whole point of the exercise, the supplier is still troubleshooting.

Bearing housings, connecting rods and pistons have been made and happy to report the crank turns freely when all is assembled. I didn’t really think out the mounting as now the big end bolts hit the deck on BDC and there also isn’t really space for a decent sized flywheel. I am going to make 2 spacers to gain some height and hope they will not look too much as an afterthought (which they are).

I have yet to fit the centre bearing which will also axially locate the crankshaft

Andrew
17 May 2026 at 14:02 #848619
AStroud
Participant
@astroud
I managed to solve my clearance issue by making a recess for the big end cap bolts and using socket head screws. The centre bearing is assembled and this sets the crank axial location and it turns over without any binding which is pleasing. I have fitted a flywheel and made a wooden base.

Next will be the rotary valve, I have in mind a valve turning at half engine speed but want to mock it up before cutting any metal. Still contemplating sculpturing the cylinder head, possibly simple channels to pick out 4 raised squares on each side.

Andrew
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