Large Model Association Forum

After 4 short flights,the silencer face seems to have melted,and the resulting uneaven face on the silencer has pitted the face of the engine.The silencer bolts were captive,the bolts were wired,and a gasket was fitted.The silencer is a Pitts type and although not very heavy has vibrated sufficiently to cause friction welding to take place.The silencer was unsupported,it will be next time.Advice please,do I use a gasket?,I cannot get both mating faces flat,as there is not enough metal on the engine,therefore I need a filler of some type,ie plastic metal,any ideas?

Name of engine Barrie ? The pitts silencer should not melt with a normally running engine. Period.
I have a theory about using exhaust gaskets which I put into practice. The ends of the exhaust flange
when tightened will compress the gasket more than at the centre resulting in bowing eventually. It will
then vibrate loose.
My system is to very carefully face the silencer and engine exhaust faces.so they are
dead true and then use a very high temperature silicone sealant. Just a smear and
your exhaust will never leak or come loose again.
In my time working on Ford engine testing The commercial engines had an exhaust manifold fitted
with no gasket. Just just perfectly faced surfaces.

Tony.

Tony,Thor 45cc,30 minutes running rich on the ground,plus 4 flights of about 10 minutes each over two days.I cannot fully face the engine flange,the indentations are too deep.I have a tube of 700 degree c silicone,I was thinking of cleaning the mating faces with a file and using the silicone to bridge the gaps.

Barrrie if you get the flange as true as possible and the indentations do not run from the exhaust port inner
to the edge of the flange I think you will get away with it because the silicon will fill the recesses.
Running rich may cause some vibration but not overheating.
Just make sure that your ignition timing is correct.

Tony,the timing may be out,I will recheck it.I changed the senser and unit to a Rexal and also changed to a CM6 plug.Thanks for your help

Barrie B

Barrie, the ignition should fire just as the magnet is exiting past the sensor.
How do you intend to to determine how many degrees advance that is?

Barrie, the method I use to secure my pitts silencer involves a bit of technowelding.
The enlcosed pictures are self explanetary. It really isn't very difficult. Just research on youtube
and do a little practice first. Thread the rod one end and make a shape to fit the silencer on the other.
use a clean stainless steel brush to prepare the Aluminium. Make the bracket from stainless steel if you can.

Tony,I have just come home,hence my lack of response to you latest posts.The timing should not be much out,I am using the same mounting holes as the original sensor,so it is likley to be within the slot adjustments.The engine starts easily and runs well.I was going to attach the silencer to the firewall,but I like your idea,if the engine moves,the silencer goes with it

Hi again Barrie, just a thought. Not all ignition systems are timed the same. Here is an extract from one of the forums which is relative to your engine ignition system. Only one small thing - the thor 45 manual states the timing for the original ignition as 12 to 14 degrees not 18 degrees BTDC.
Retarded ignition wil cause your engine to overheat Barrie. It shouild be 28 to 30 degrees for the rcxel unit.


Hi Just a thought .You have taken off the Heath Robinson advance /retard fitted to the Thor havent you ? It used to be connected from the throttle lever and moved the Hall sensor when the throttle was opened . Brilliant idea back in the dark ages but old hat now .The modern CDI units have an built in advance /retard system that is really good .
john melia 1 01/09/2014 12:35:14
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It didnt have one of those on , its a new up to date engine , and came with thors cdi unit which is basically the same as a rxcel unit , only the timing was 18 degrees btc as opposed to the rcxel being st 28/30 degrees btc .

It runs perfect now since we changed to a rcxel unit and re-timed it to 28 degrees . The original thor cdi was faulty as was the hall sensor , all good now though

Hi
I had the same problem with a OS 33 petrol ,the silencer kept coming loose .Despite drilling the bolts for wire locking and the use of spring washers. I was told clean it up and glue on with epoxy.( still using the bolts of cause ) To be honest I was not expecting it to work, but i does. No more loose exhausts.
Just in case any epoxy should get into the engine I put a small peace of tissue paper into the exhaust port, this will blow out when the engine is started. Hope this helps.

Thanks for your help,both of you.I now know the cause,the timing was 12-14 degrees (I had used the original sensor mounting holes),but for the Rexl unit it should be 28 degrees.I have filed the mating faces and now making a bracket to support the silencer.

Barrie B