Too much compression? I think hot plugs are more fragile and that could stress them out.
Rusty

Alternate Theory: It's a Merco...is it self destructing and making metal? If so the chips/flakes may be getting into the plug coils and shorting it out. I had a Merco 35 once, this happened. Really like my Merco 61's though.

Might be something as simple as adding a head shim, to reduce compression just a bit...couple thousandths should do it? 

...is it self destructing and making metal? 

Little metal particles cause plug filaments to break.  I don't know the mechanism, but they do it.

If you are going to toss the Merco, then make sure you toss it in my direction!

Andew.

Might be something as simple as adding a head shim, to reduce compression just a bit...couple thousandths should do it? 

Hmm, since the original head shim is a fibre ring and eminently crushable its best to use alloy Fox 35 ones - that and I wonder if the reach on the plug is too long?

On further inspection of the head, I can see very small metallic flakes..oh dear oh dear, is this motor doomed? After all the time and effort getting a muffler sorted for this engine. It was given to me recently as an unwanted item..I'm beginning to see why.
Phil

...and when you try it again, don't repeat the 12,000 rpm with a 10x6 prop experiment. For a Merco 35, that's just wrong. From a stunt perspective it proves nothing and may have been a factor in the metallic flake production.  

Very likely there would have been at least one. Regardless, adding one may solve the plug issue. I'd try it.

I made some workable aluminum .015" shims out of a sheet of K&S aluminum with a compass with an exacto blade affixed to it. They were for my Thunder Tiger 25 and worked great,
Rusty

Thankyou for all the interesting and helpful replies. Its much appreciated.
Phil

Thanks for the tip Randy, yes I have got a gasket under the plug. I didn't know the problem with electric starters, tho' I don't use them much.
Phil

Steve suggested that I put in my two cents worth. Well I have run lots of 29 and 35 Mercos over the years and they are good stunt engines. They have a horrible sort of red fibre  gasket between the head and the barrel. I don't know quite how thick it is, because you always seem to destroy it if you take it apart. My guess is that it is at least 30 thou thick. I got some grey gasket material from an auto shop in Cambridge Ma. It certainly works well, but it is probably a little thicker than the original.
Much better that you use some thick aluminium gaskets and see what is the best thickness for your engine and application. I think that the gaskets for the Fox 35 and or the OS Max S do fit, but please check on that, as I am not 100% sure on this. As far as metal bits flying around the engine, there are two possibilities. The first is that the piston baffle is hitting the head. I would think this a little unlikely. The second possibility is that when the first plug failed the wire broke up and that, I think, is the source of your problems. The bits are not all ejected through the exhaust. You change plugs and bingo, one of these pieces clouts the element and another plug ruined, with more bits of plug wire flying about. You carry on changing plugs and then getting a failure.
I had this very same problem years ago, but this time with an Enya engine. The plug blowing stopped when I took the engine to pieces and gave it a thorough cleaning. I even found some metal that was stuck to the top of the piston! Just give your engine a thorough strip down and clean. That should stop the problem.
Sounds as if Balsa Butcher doesn't like Mercos. The very early Orange head versions together with the more common red head variety are good 4-2-4 stunt engines of the old style. As good as anything in the US at the time. The Black head versions can be problematical. I think these were produced by another firm (Forrest Engineering?). These can have tight P/Ls and can wear rapidly. The firm seems to have used different metallurgy over time and some at least are extremely good. Others can be like Enyas and need a long break in. Hope this helps,

Andrew.

Andrew, thanks for your thoughts on this issue, the little bits of metal that I did find on removal of the head were tiny flakes, ie, flat in shape and very small, like a grain of salt size but flat. And with ref: to what Steve mentioned, I took the back plate off and found some scrape marks on the plate. Now, I've never stripped an engine before, Been model flying  5 years now, and this will be my first strip! Woo Hoo! I'm eager to do this perhaps with a few pointers from you chaps on here as I go along... I will have the crank pin looked at by someone in my club, but a strip down and clean sounds like a 'must'. I hope to start this at the weekend ( I also have a septic tank to pressure wash but thats another story!) but will come back here, if thats ok,  when I need advice. The first thing that has me puzzled is how do I remove the piston?  By the way, this was a 'Blackhead' but the enamel is long gone, The head fins are finished off square as opposed to being slightly rounded off as with the earlier? orange head. I'm also having a new venturi and nva made as this engine is an  r/c type. I would like to save this motor as Its seems perfect in my Flite Streak (when its not burning plugs). I have a feeling 'I'll be back' to coin an Arnie phrase.. Thanks Phil

Hello Chris,
I know that 30 thou sounds very thick, but have you ever got one off? I stand to be corrected because it is a long time since I had a gasket off, but I remember being amazed how thick the gasket was. I will ask on Barton Forum, I am sure there are folks around who will know!
Back to the metal bits problem! I am a bit embarrassed to have forgotten the crank pin rubbing on the back plate. A good idea to put in a thick gasket at this point. Better still, if there is much fore and aft play in the crank shaft, then I would be tempted to put in a thrust washer behind the drive plate.

Andrew.

Hi Chris,
I took a look at one of my old Redheads that needs some P/L work doing, The head gasket was just as I remembered. It peels off layer by layer. I think I will stick to my 30 thou estimate or even up it a bit!
You are quite right about the baffle being an unlikely customer for coming together with the head. However we now know that it was bits of alloy from the backplate. So it looks highly unlikely that I will be the recipient of another Merco!

Regards,

Andrew.

If the head was installed backwards, then I'm sure the baffle would contact the head. With the head fins apparently altered, all bets are off on the engine's virginity...meaning that you don't assume it was put back together correctly! Consult with your clubmates about the direction the rod goes in, because that may make a difference, and maybe it's backwards. 

One thing to remember on re-assembly is that the baffle goes on the bypass side, along with the lower of the two ports in the cylinder walls...the higher is the exhaust.  Steve

PS: Good fun with the septic tank!

I found the video.

Thanks, but I can't take credit Phil. That's not me, I just searched Youtube to see if I could find the video you mentioned.

Rusty, I was impressed that you were able to bring up the video onto SH. I don't know how its done. Phil

Oh. Duh! I get it.
Youtube recently changed the video sharing look, so I'll get back to you via PM with a screen shot of what to look for. For now, you can also just copy the URL straight out of your browser's address bar and paste into your message. Some forums will make a video window like the one above. Most just show is as a hyperlink, but it works just fine that way too. I'll get back to you later with the better way.
Rusty

oops! forgot to say the image of the head shows where I filed down the edges to the flanges, and I also meant to post a pic of the Flitestreak the Merco is in. Hopefully its here. I was lucky to get this well finished model from a flying buddy so I can't claim any credit for this one! If only...