SUPER TIGRE 5cc BUILD

Part one by Ramon Wilson

Starting the crankcase

I enjoy producing 5cc versions of 'diesel' model aircraft engines. The one chosen for scaling up this time is a small 1cc engine produced by Super Tigre in the 50s. ST also produced 1.5 and 2.5 cc versions on similar lines - i.e. rear intake drum valve induction with bolt on ball raced front housings - single for the 1 and 1.5cc and, I believe, twin for the 2.5cc.

Why 5cc? well for me it's a good size to handle, not too small to be fiddly especially on the smaller parts and not so big as to require sourcing larger lumps of material. They swing a decent sized prop and make a lovely sound, too. I know there can be some problems with sourcing fuel for some, but for me no glo motor gives off an aroma like a diesel does.

Materials are reasonably basic - He 30 ali for the case, front housing and head and En1a leaded steel will be used for the liner, with a cast iron piston and the crankshaft made from En24T.

Making these engines is not a long term commitment. They have a low part count but the machining does have to be accurate with good - and in the case of the piston liner fit, exceptional tolerances but it is all achievable with easily made kit and when it bursts into life at the end you have your reward.

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Decided to do two cases in case of any mishaps. That way if anything does go pear shape you don't have to go all the way back to square one unless of course it's pear shaped for the second time round.

Began with a couple of blocks from 65mm square bar. I weighed them just for interest - 555 and 560gms it will be interesting to see just how much of that will become swarf.


These were roughed down to block size plus .5mm on all faces using a small (approx 30mm dia) flycutter.
The centres for the cavity and the liner were accurately spotted relative to each other...
...before setting them in the four jaw to bore the cavities and drilling and reaming for the rear bush.
The front face was skimmed to ensure squareness to the bore then an area which will cover that of the front housing taken to finished size (i.e. 0.5mm deep) the depth of the cavity being from this recessed face.

Proceedings were held up somewhat due to swarf from the previous week’s chip producing effort finding its way into the cross slide drive gearing and locking the leadscrew, which meant a strip down - twice  - of the cross slide to clear it.

After making up a spigot for locating the bore on later milling ops the blocks were set up and 4mm pilot holes drilled and reamed at the intake location. These will be used for setting purposes later before being opened up to final size. The position of the intake hole relative to the valve sleeve bore and the inner face of the cavity was marked out as a double check on position but the actual positioning was all done from coordinates.
Itching to get on,the first block was set up on the lathe for boring out the top face through to the cavity for the liner.
Although I was aware I had dimensioned the depth of the transfer ports and drawn the bore straight through it soon became clear I hadn't thought this through fully - (oops!) Not giving this too much thought then I'm afraid I blindly ignored the interupted cutting feedback - it was difficult to see till the bore had increased - thinking it was the breaking through into the sides of the cavity  . The penny dropped too late of course before it was realised that I was eating into the back face of the cavity. Well after the usual 'Deary me, look whats happened' kind of phrase  (yeh you wanna believe it ) I could see the error of my ways and decided to break it down to see what it looked like before going any further. Taking a good look at what had transpired it looks as if it can be saved by setting it up and boring out for an insert. This is now very definitely 'Case # 1' to be used for all set ups. Set the second case up and bored to a very definite 21.0mm deep which was a big improvement.

So an inauspicious day ended with these ...

Reclaiming the case began with making an insert like a thick washer about .5mm thicker than required. The hole (in the insert) was bored to 9.95 and the outer rim cut at a half degree angle so that as it would be pressed into it's recess (and Loctited) hopefully the join would be near invisible. Once this was done yet another expanding mandrel was turned up - I just love making these things, they work so well.
Once the Loctite had cured this was faced back to the original depth then the case set up as before for boring out for the liner. Finally by using the chucking reamer by hand from the outer end that .05 left in the bore of the insert was reamed inline with the original bore. The reclaimed #1 is on the left.
Back to square one rough milling the outer faces could begin and this was done in the vice with packing to protect that finish turned top face. First the rear end was reduced each side to a 'reasonable' finish as this was to be used for subsequent setting up then the sides above and below the lugs were taken out to within 1mm of finished size.
Now the finishing could begin so first off was to mount that spigot previously machined on an angle plate and square it up on the mill. The mill spindle was centred accurately about the centre line of the spigot
The case was set square to the table via that reduced width then the lower (actual - the top in the shot) face was then milled as a datum and the lower faces of the lugs and crankcase width finished off to size.
The case was rotated for the sides above the lugs and the lugs themselves to be finished this time using an end mill which had had the corners radiused. The top edges of the main body sides were then milled before setting up again to radius the edges with a corner rounding cutter ......