View Full Version : Small engine building

03-10-2005, 09:13 PM
Hello all.
I have drawings and castings for an air cooled opposed twin 4-stroke. It's a Coles kit. 1 1/4" bore, 1 1/8" stroke. It was given to me from a friend.
There will be a few changes I'd like to do to the design. Mainly to make it more compact and reduce weight so it can pull around a 1/3 scale piper cub.
Anyone here build small i.c. engines?
I have a few questions.
The crankshaft material...I plan to harden and grind. A-2 suitable? What would be best?
Balancing the crank, what kind of a formula would I use to figure this out?
Cylinder liners... Hardened steel (lets say A-2 again) with cast iron rings?

03-10-2005, 09:36 PM
Is that the Wall Wizard 45 CC engine?. Nice gift if it is, castings go for $379.50 these days.

I would recommend not to use hardened steel for the crankshaft, 1045, or my favorite, Stressproof 1144. Balancing should not be a problem with this small a crankshaft.

For the cylinder liners, cast iron. Durabar is an excellent material for both the liner and the rings.

Have fun.

Jim Luck
03-11-2005, 01:02 AM
4140 pre-hard would also work for the crankshaft, its 28-32Rc, and very tough. 1040 of 1045 can be hardened at a very low temp. just don't let the heat soak thru or it could become slightly brittle, you only want surface hardness. C.I. sleeves-YES!, Enjoy Your Project--

03-11-2005, 01:06 AM
philr,jc pretty much covered your question.i wouldn't be too concerned with weight,most aircraft end up tail heavy anyway.but if you want to save weight,could you use aluminium for the cylinders,and put in a liner?i don't know if metal spraying is done in your area,maybe you could use a brass liner with chrome coating.thats what most manufacturers use.aluminium piston,and cast iron ring.be aware too,that most home-built engines are generally meant for display,rather than use in aircraft.you may get carburation problems with engines not designed for this purpose.having said that,it sounds like an interesting project.keep us up to date please.i have plans for a .60cu.in. single cylinder 4stroke engine,the Matador,for which i'm slowly collecting materials.the plans also include conversion to a 120 twin.


03-11-2005, 07:33 AM
I'm no expert, but it seems that crankshaft balance on an opposed engine takes care of itself as long as your pistons and conrods all weigh the same.

J Tiers
03-11-2005, 08:48 AM
It can, assuming the pistons have opposed movement.

A 4 is easy. A 2 cylinder would need split or bent conrods (like Johnson Waterbug) to get the pistons opposed.

Otherwise, there is a "couple" due to the pistons being offset. They would need a weight to oppose that.

03-11-2005, 01:58 PM
I have an old Saito FA90-T opposed twin which has a single throw crank (one cylinder is at TDC whilst the other is at BDC), and however they balanced it they got it right. Sure is smooth. Sounds neat too with that goofy firing order.
A Google of... model engines "crankshaft balancing" ... came up with this....
Which may be of some help, I'm sure I've got an article or two on the subject by Carence Lee or that Cunningham guy out in the garage somewhere but I wouldn't know where to start looking.

03-11-2005, 02:36 PM
I wish saito made engines for full scale too.
I used to have a nice Saito 3 cylinder radial.

03-11-2005, 03:56 PM
Here is a site that has alot of info and links about small scale IC engines. Looks like this guy even made his own mandrel bender and some other tools too. Make sure to watch the video.


03-11-2005, 09:29 PM
Thanks for the replys, glad I asked here.
Yes it is a wall wizard, 45cc sounds about right.
Stressproof 1144 comparable to P.H.4140? Lots of P.H. 4140 down at work.
Is durabar an easy material to find?
It would be nice to have the cylinders plated. Sounds expensive though. I'm sure there would be a place that does that sort of thing not too far away.
I had a look at the page on balancing. One method mentioned was to use the weight of rotating parts + 50% of recipricating weight. So I suppose the crank pin and a portion of the connecting rod would be considered rotation weight?
For an opposed twin, half of that weight would be used since one side of the crank pin is already balanced by the other half of the engine?

03-20-2005, 10:03 PM
What grade of cast iron should I be looking for? Is Durabar sold in Canada?
sorry for all the questions.
Haven't had much time on it yet since it's a spare time project between school and work. At least I can say the pistons are done.

03-20-2005, 11:22 PM
phil,for the cast iron,look in recycling yards for old window sash weights.many of these were of nice fine iron.i got all 6 axleboxes for my 5"gauge loco out of one(admittedly 40lb)window weight.otherwise go with the durabar,assuming you can find some within a reasonable distance.