Excellent tip 74/7!

I hate butchered backplates on PAW's and the like. You see this quite often on the second hand examples appearing on eb*y.

To update:

Last night I felt a renewed sense of resolve, and I retrieved the engine from its hydrocarbon bath (where I had left it in disgust)and drained the solvent (some old weed eater two stroke mix) from it.

I had a length of hard plastic U-shaped conduit extrusion in the scrap bin. It is square U in profile and the top part (to cover the wires within) snaps on. It is used for computer cabling. I recently helped myself to some scraps from my workplace. Took it home .

Now, the backplate on the Aurora is well thought out. On PAW's, the milled slots are radiused inwards and they are a * $9%t * of a thing to get off without the correctly curved tool. A straight edge on a PAW backplate (at least the backplates on the one's I have) won't do the job properly.

Not so here. The slots on the Aurora backplate are milled straight across the flange. So, not wanting to damage the slots, I butt the backplate flange up against the plastic lip on the conduit off-cut ... and I have a win! The lip fits perfectly and the plastic will not deform the aluminium!

[8D] Cool ... with a quick twist, it unscrews without a hitch.

Now, the thread on the blackplate recess is quite deep, and it is extremely fine, but well formed. The mating thread in the crankcase is also nicely worked. The backplate screws on and off the crankcase very smoothly, but beware ... cross threading would be just too easy, so I make a mental note to go about re-assembly slowly and gently. I will use a little air tool oil on the threads when I do it back up.

As for the gasket that seals the backplate to the crankcase, well it is a black rubber filled fibre affair, and is hand cut to a round shape! Roughly done; but it is "OK" for the job, and it hasn't beem torn or damaged by the removal of the backplate. I think about removing it, but there is no need, so I leave it there.

There are some slight concentric marks on inner surface of the backplate, from where the big end has rubbed against it (no doubt from my earlier efforts when turning the engine over). I may scource a sightly thicker fibre gasket when re-assembling, like the type used on tap (US faucet) fittings, as maybe there was a slight binding there ... or before re-assembly, I might just relieve the backplate a little using the "flat glass and oily 600 grit glasspaper" method. The first option will be easier. I'll see.

Peek inside ... OK, if you've ever looked inside a PAW, that's pretty much what you see. Crank and pin are unremarkable. Conrod is a steel tubular affair and the little and big ends are both pretty hefty! Bronze bushed. This conrod seems to be way, way over what is required for a .35... but it is clearly made strong to take the tough punishment that only an over compressed diesel can mete out.

It is hand finished, and filing marks are very evident. Not surface finished with any great aesthetic finesse at all ... workmanlike is the word! I think I will finish smooth this item off myself.

I laugh when I see how clearance on the outside of the big end to the crankase has been "hand" fitted (i.e. filed!) to tolerance by a series of flat swipes with a file. Very "hand made", very old school manufacturing! But nonetheless perfectly effective. This has not been made by a CNC robot! Hey, lets face it, it does not need YS fuel pump tolerances !

The steel liner is thick. It has 4 intake gas transfer ports radially milled around it, to allow the incoming charge up into the combustion chamber. I can't see too clearly, but looks to me just like a PAW-type gas transfer set up. It has a very substantial iron piston as well. All looks well enough made and well put together.

Looking around inside the case carefully now ...

No surprise - there is obvious swarf ... fairly fine though ... so I slush what I can see out with a few quick dunks of the bottom of the engine in the petrol bath, and then carefully wipe out what I can get to down through the backplate entrance.

Gosh, its still "rough" to turn over .... so I inject some air tool oil (a healthy squirt) in the intake port (I have the carb off) and around the main bearing through the back , and turn it over a couple of times. Oil mixes with some dribbles of petrol still in there. I tip the nose up, and a metallic sparkly glistening sludgy mix of oily petrol flows out. The oil has "floated" out much more of the swarf!

Bearings feel really, really lumpy. I suspect they may be "cream crackered" (US: trashed) by the machining particles that were there. A lot could still be in there. Or maybe the bearings are just gummed up, if I am lucky. Can't tell just yet.

I tentatively turn it over (after trickling the oil out), and it feels a bit smoother, the air tool oil still in there has helped a lot.

I decide ... No question about it, it's getting a full strip down. A mechanic friend has a puller to get the prop driver off ... bearings must come out, rest will be easy.

I'll do it on the week-end I think

To be continued ...