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Mills 0.75 piston and conrod replacement

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Ian McTaggart26/03/2019 21:12:06
17 forum posts

I'm one of those time-warp returners: - after 50 plus years. Thank you in advance for your indulgence.

I have a couple of Mills 0.75 motors, one of which runs well but the mounting bolt holes are rather badly damaged. The other has suffered the fairly well documented catastrophic failure which has resulted in the gudgeon pin being ripped out of the piston skirt. Cosmetically it is excellent.

What do the experts think about swapping the good cylinder, piston and conrod into the better crankcase/crankshaft? Will it work? Are there any tolerances that may be out?

Thanks very much.


Jon - Laser Engines26/03/2019 21:35:37
4888 forum posts
183 photos

Seems like a good plan to me. If you are really worried you could take the crankshaft, rod and cylinder over as then all of the moving parts are friends. If however the other has a better crank then stick with that.

As long as you dont thrash the pants off it right after surgery i cant see any reason for any problems

Simon Chaddock26/03/2019 23:31:05
5483 forum posts
2878 photos

Are both original Mills?

If so as long as you keep the matched piston, liner and contra piston together, on an original mills they were a lapped fit, it should work fine. If they are Mills copies it might need a bit of care as the con rod lengths might be different.

The only issues could be the fit of the con rod big end over the crank pin. This is a known wear point in a Mills.

brokenenglish27/03/2019 06:43:21
468 forum posts
29 photos

If the engines are more than 50 years old, they're originals, so there won't be any problem.

As Simon says, just be careful with the reassembly of the big-end, because this is the only mechanical "interface" between the two different engines. If the big-end assembly appears a bit "doubtful", then try changing the crankshafts as well.

Just make sure you install the cylinder the "right way round", transfer ports at the front and induction ports to the rear!
It's easy to get them 180° out, which prevents the engine from running of course.

When reassembling, it's a good idea to cut a small strip of 1/16 ply, to match the width of the exhaust ports, then insert it through the ports, across the top of the piston, to prevent the cylinder from rotating when you tighten the head down.

Peter Miller27/03/2019 08:16:13
10314 forum posts
1231 photos
10 articles

Yes, as has been said the piston cylinder fit is the critical bit.

Mills Tolerances in other areas varied.

Many years ago a chap wrote to me and asked why two of his Mills engines were more powerful than the third.

I told him to check the sub-piston induction, the size of the gap under the piston skirt at top dead centre.

The leas powerful one had hardly any gap while the others had much larger gaps. The weaker one strated as easily anmd ran as well but just did not deliver the same power

Ian McTaggart27/03/2019 15:22:03
17 forum posts

Many thanks gentlemen. That is most helpful and reassuring.

John Duncker27/03/2019 20:18:51
79 forum posts
7 photos

Keep a close eye on the conrod

You might find a spare on Ebay but an evening with a pillar drill some hard ali and a set of files will turn one out.

Lots of info here The Mills story

Ian McTaggart31/03/2019 22:23:05
17 forum posts


Many thanks for your advice.

The transplant is complete although I haven't run it, it feels and sounds good.

I bought an 'old people's' bottle opener off Amazon which was excellent for getting the cooling fins off without any damage and I used my Dremel to cut a couple of slots into the end of a tube spanner, which was perfect for removing the backplates. The crank in the broken, recipient engine, felt nice and tight with very little end float so that has stayed and the donor conrod looked snug on its new big end.

For the serial number officianados; 58 121 has donated its cylinder, contra-piston, piston and conrod to 50 968 which I think must have been damaged very early in it life.

Hopefully it will run ok when it is re-installed into the restored/rebuilt Frog Tutor that has been the donor's home for the last 50+ years.

Thanks chaps.

Engine Doctor01/04/2019 08:45:52
2330 forum posts
28 photos

Hello Ian I'm sure it will be fine . I had a Mills p 75 made up from bits and pieces from a few Mills and it ran really well . Enjoy your return to the hobby.

Simon Chaddock01/04/2019 09:06:11
5483 forum posts
2878 photos


I would always run a 'rebuilt' engine on some sort of test mount before installing it an air frame.

I felt jhe process of flick starting a diesel put a significant strain on the engine mounts so tried to get all the appropriate settings tested first. Even a Mills does not start that well if things are too far out. wink 2

I was quite happy using just a piece of floor board with a suitable notch cut in it and 4 No 6 wood screws. Indeed I have run my Mills just holding the crankcase directly in my hand but I would not recommend it!

Do let us know how it goes.

ken anderson.01/04/2019 09:14:21
8474 forum posts
773 photos

sad …. i tried the floorboard method/woodscrews,they unscrewed themselves and the motor headed for the dining room window...thankfully running out of fuel before kulou and a divorce threat....

ken dept.

Bruce Collinson01/04/2019 13:25:52
409 forum posts


Ought you to have unfastened the floorboard from the dining room floor first?


Bruce Collinson01/04/2019 13:26:11
409 forum posts


Ought you to have unfastened the floorboard from the dining room floor first?


ken anderson.01/04/2019 16:55:18
8474 forum posts
773 photos

smiley...outdoors bruce….one of my flying friends did run his OS 70fs inside when her indoors was away shopping as you do/not.....there's an existing thread going about all our naughty goings on when the boss is out etc..

ken dept.

Martin McIntosh01/04/2019 18:17:31
2970 forum posts
1090 photos

Whilst we are talking about Mills, does anyone out there have a spare compression screw for a .75? I have original .75 and 1.3 Mk 1`s from around 1946 plus a later .75 and 1.3. Only one compression screw to share between the .75`s.

Ian McTaggart01/04/2019 20:00:05
17 forum posts

Good suggestion to run the motor and get the settings right before reinstalling in the plane. Thank you.

Fortunately I recently found my nice 1960's aluminium adjustable engine clamp in the back of the shed and after marinating in WD40 it has cleaned up nicely, so the floorboards safe!

Ian McTaggart02/04/2019 18:18:38
17 forum posts

She's a runner! Thanks very much for all the encouragement, without it I would still be cogitating.

J D 802/04/2019 19:59:17
1317 forum posts
78 photos


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