rc plane crash - 68% pitts challenger robbie skipton at raf cosford

[Gary Manuel]

Robbie,

Just a thought. Is the plug cap one of the proprietry ZDZ types with the grub screw?

Maybe the plug cap part is somehow interfering with the plug to cylinder head contact area, allowing it to unscrew?

If so, it might be an idea to switch to the more conventional plug cap / spark plug.

This is the type I'm on about. As you can see, this had been running VERY rich when I bought it.

PS - very much looking forward to your build log, if you decide to do one.

[Frank Skilbeck]

On racing motor bikes, they use wire locking to stop nuts and plugs from coming loose, is this an option here. Or failing that some form of tab washer, but that would make the plug stand off and hence lower the compression ratio slightly.

[Robbie Skipton - Skip Model Designs]

The plug system on this motor is not the one pictured above. It is simply put plug in a torque up.

I cannot use wire on the plug as above due to the caps.... alreadt thought of this methord

[Owdlad]

Glad you mentioned that you torque the plug up. Is it around 7-9 lb ft ?

I believe the problem is to do with expansion as Aluminium expands roughly twice as much as Steal.

Have you tried to re-torque the plugs up at operating temperature ?

[Bob Cotsford]

Google says that they are taper thread plugs, not the straight thread types we are all more familiar with. I can imagine these going quickly from a tad loose to goodbye plug. If you throw away the warranty is it worth getting the heads bushed and tapped for straight thread plugs?

[Denis Watkins]

Just on taper seat guys, both taper and flat seats are parallel threaded.

What changes is the plug seat that originally was flat on our Ford Anglias, but designed tapered to increase the gas/seat surface area for a better seal, so Robbie's motor had the improved version.

But neither are interchangeable, so just the same, you would not get a good seal with a flat seat plug in a taper seat motor

[Geoff S]

It puzzles me why losing a plug on one cylinder should cause loss of vital crankcase pressure on the other 3. The only multi cylinder 2 strokes I've had much to with is Scotts and the crankcases are completely isolated from each other as they are on the Scott look-a-like engines and motor cycles manufactured by a friend of ours a few years back (Silk 700s). In fact George Silk was an ace 2 stroke tuner and one of his must dos was improving crankcase compression which he did to great effect. I know plugs whiskered regularly on my 1932 TT Replica and, if I guessed right, I could change the whiskered plug with one of the cleaned ones I kept in my pocket without stopping the engine - or even getting out of the saddle

So on the ZDZ does each cylinder share the same crankcase volume somehow? Not trying to stir things up. I'm merely curious. Or did I completely misunderstand?

[Paul Marsh]

I asked Robbie why losing one plug would lose the engine - it shares the same crankcase pressure. As it's a two stroke, when that happens, all pressure is lost and fuel shut off. If it was a flat four 4 Stroke, it would run on 3, just a little lumpy on every other stroke (when the dead-side cylinder fires).

That's why it stopped.