Mills 0.75 compression screw.

[Martin McIntosh]

Compression screw needed to fit original Mills 0.75 (4BA).

[Don Fry]

Martin, as a functional alternative. If you want original, look elsewhere.

But all a competent lathe owner needs is a hard steel 4 BA bolt ( that is a bit of good steel with a thread that goes most of the way to the head, (but leaves an overlong shoulder of uncut steel). Then is just a question of you supplying (or specifying) the length of the threaded section, and how far above the thread you want the hole drilled for the tommy bar.

And a bit of a clue as to what the bottom looks like where it goes into the contra-piston.

From memory, 30° taper, bottom about half diameter? 2mm in BA. Distant memory, did the conta-piston have an indent in the centre? Hence a taper.

An owner of this compression screw can supply?

Before you ask, I am a lathe hacker. I need stuff in front of my eyes to cognate.

But there are competent users on this forum. There is or was a bloke, Scottish?, who offered engineering services.

You might want to open the thread up to a wider audience.

[Andy Stephenson]

You don't need a lathe. I made a replacement compression screw for a PAW 55 by taking a longish 4BA screw and with a part of the threaded end in a drill. I reduced the head and some of the shank with a file until it was thin enough to bend. I bent the thinned end over at a jaunty angle and job done.

[Martin McIntosh]

Thanks for the replies. I have tried making my own (have a lathe) but don`t have hardened steel. Also, the end needs to be like a grub screw to provide sufficient contact pressure otherwise it unscrews in flight. The motor in question is fine with the correct screw from my other 0.75. I do have a PAW 55 and wondered if they have the same thread. Must try it some time.

[Andy Stephenson]

The reason I made a replacement screw was that the originally one supplied kept coming loose and it didn't have enough thread to allow a locking lever which I made from a sliver of steel sheet. This kept it tight. I suspect that the thread in the ali head had worn loose because of the amount of running it had had and not because it didn't have the a grub screw end which the original one didn't either. When it was new this wasn't a problem.

Edited May 19, 2021 by Andy Stephenson

[Simon Chaddock]

The fact the compression screw is threaded through the ally head suggests you shouldn't use a hardened screw which will wear the thread in the head. Indeed a softer steel is likely to locally distort on the face of the hard contra piston which will increase the contact area so resist any tendency to unscrew when running.    

[Andy Stephenson]

Simon, I agree about not using a hard screw but I think it's more important not to use one with a knurled and cupped end as this will just grind into the contra-piston each time it's turned. The best solution is to use a locking lever.

[Engine Doctor]

No need for any special treatment for the end of the comp screw . The Mills contra-piston is hard iron and will not be affected by the screw . I have a few mills and all have just normal screw ends ( not polished ground etc ) . If Contra-piston is a good fit it wont come undone in flight. 

The top of the screw is just like a normal  4 BA screw with an untapped part approx 4mm long . This is drilled and tapped to accept what looks like a 8 BA screw with a long plain shank. All simple stuff and is easy to make . Once made it can be heated and cooled in oil to get a nice blued effect as the original had. The small stop post is again just a short threaded  8 BA  screw with a plain shank .

A nicer looking comp screw can easily be made from a 4 BA cap screw then bent as per picture