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I F Allman in 1/3rd Scale.

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JasonB14/06/2019 20:27:54
15510 forum posts
1594 photos
1 articles

You have seen odd bit sof this engine in teh workshop progress thread over time and with teh Forest engine running well I thought I may be on a bit of a run. So armed with a new cylinder of propane thought I would give this one a go to see if it had gone back together OK after painting.

It took a while to find the sweet spot. First of all it had a tendency to fire too soon which I put down to too high compression. Now I may only have a green dragon machine that some say won't turn concentrically and only have a cheap brake cylinder hone not a smelly proper Sunnen machine yet with just one vitron ring fitted I was getting very good compression infact almost too much which resulted on it firing every 3 or 4 cycles and petering out usually ending in a backfire that sent it violently into reverse before stopping.

Then I had a enlightened moment and remembered that when building the engine I had found that there was less compression when the primary exhaust was open as the piston does not cover it until part way towards the head so took out the plug and gave it a try. Certainly ran with a loud pop and threw out a few large flames at crotch height surprise. However it still did not fire on every stroke and seemed weak and would again slowly die but at least there was no backfire.

Finally found the sweet spot with the primary exhaust plug left slightly loose which acted a bit like a decompression valve and away it went. It's always nice when you can choose when to stop an engine rather than snip the video short just before it comes to a halt by itself and just show that its not what you have in the workshop that matters but what you do with it and subsequently comes out the door devil

Another big thank you to Graham Corry for making the patterns available again and for the help and encouragement along the way.

Ron Laden15/06/2019 07:54:14
1096 forum posts
169 photos

Very nice Jason, runs really well. I like the inverted cylinder and seeing the piston in operation.

It looks to have plain bearings, how do you decide on which type of bearing to go with on an engine. I,m guessing you must base it on what power should be available for the type of engine against drag/friction etc.

I like the black colour scheme also makes a change from green and red, what paint do you use on the engines they always seem to have a good finish.


JasonB15/06/2019 10:41:11
15510 forum posts
1594 photos
1 articles

Ron the choice for me is usually dictated by whet was on the original and as most of what I make are older engines then plain bearings are the usual option. I did use ballraces on my flame licker for reduced friction and the Forest also used them as it was destined to work that way too, had I planned an air/steam one from the start then plain bearings would have been used. Also a couple of the model aero engines have ballraces though not all with some again having plain.

having said that these couple of videos will show that plain bearings don't drag that much, first is plain, second ball races.

As for paint although I have a good collection of airbrushes and small spray guns 9 times out of 10 I will just use spray cans. On this one some of the rougher castings had U-Pol "High 5" which is a high build filler primer, the non ferrous had U=pol's "Acid 8" etch primer then all had a coat of Halfords grey primer followed by their Satin Black and finally a clear coat in this case a fuel proof satin.
Ron Laden15/06/2019 14:02:36
1096 forum posts
169 photos

I see what you mean Jason, the Allman is very free running considering plain bearings, what tolerances do you work to with plain bearings and I guess they are lapped in. The Forest is exceptionally free running but I suppose that is to be expected with ballraces, though to be honest I never imagined it would be as good as it is, I didnt think it was going to stop.


JasonB17/06/2019 07:41:56
15510 forum posts
1594 photos
1 articles

Sorry Ron forgot to answer.

Upto 6mm dia I will ream the hole so that would more than likely be with an H7 reamer which sets the tolerance though it will vary a bit depending on material and pilot hole size.

Once it gets to 8mm or larger then I prefer to bore the hole and I stop using the digital callipers as I get close to size and do the last sizing cuts by feel as I don't really have an accurate way to measure small holes to fine sizes. For bearings I would aim for a running fit that slides and rotates easily but has minimum wobble, on say a flywheel or pully then I would be aiming for something a bit tighter that needs a bit of a push and a twist to get it on..

I don't lap the bearings, just use a fine feed and sharp tool on the finishing cuts

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