Rather than get lost in Bob and Lou's Fox 40 posts, -

Timing. I agree with Lou, the baffle ST C35 is considered more stunt friendly than the flat top. Engines with a large timing lead are considered to have "run away" characteristics. To tame such motors and bring them into 4/2 mode, we reduce the timing lead. So where does that leave the flat top ST's, which have zero timing lead? According to received wisdom they should be the best stunt motors.

Rustler originated due to the T/R team I fly in, (although it's many years since we did any). Derived from Ian RUSsell and Greg SadLER, with a T thrown in for luck.
I like it so much that when I diverged from the original Merco Co. I now do engines under the name Rustler, or Rustler-Merco for stunt engines.

Timing lead is as described by George Aldrich, and is the No. of degrees shaft rotation the exhaust opens before the transfer(bypass). Less degrees, say less than about 12, = good 4/2 break. Over this, as on many racing and R/C engines, particularly modern ones, e.g. up to 20, = run away tendencies. This prompted my thoughts on the flat top ST's. They are racing engines, but have zero lead. I guess the simple answer is to run a flat top ST and check it out. Dave Day gets very good runs with his flat top ST C35 in his Box Car Chief, but I have not noticed any strong 4/2 characteristic.

Am I correct in thinking that timing lead is what is often callled 'blowdown'?

Noticed Eric Rule dropped your Rustler .40 Metamorph. It was on my list of engine I gotta get someday. Are they still obtainable directly from you?
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Jerry - Whut? He did? He never told me! Definitely still available from me, £95 inc. "International Signed For" airpost. Also 51, old type 61, and new 61, and watch this space.

I was under the impression that blowdown was the difference between the exhaust open period and the bypass port open period. Leastways, that seems to be the convention around these here parts. The OS 40's I doctor are set for 140 degrees of exhaust open and 126 degrees of bypass open for a blowdown period of 14 degrees.  That means the exhaust opens at 110  after TDC and the bypass opens at 117 after TDC, 7 degrees of piston travel later.
Don

I was always intrigued that Garafoli got away with producing so many of the 35's with zero difference in the timing of the exhaust and boost ports.  They ran just fine that way, although they had a tendency to really backfire hard when starting.  One common mod for combat was to raise the exhaust port height.  You could get more power, but at the expense of overall general handling and starting.  From what I read, the theory was that when the ports opened some of the exhaust would go into the bypass and then come back out as the charge in the crankcase started scavenging the cylinder.  Also, the cylinder configuration was patented.  If you look really close the bypass ports have two angles cut into the sleeve at the top of the port.  The little angle in the edge of the port makes the airflow through the port stick to the sleeve and curve around and up towards the head with no baffle on the piston.  It looks like a pretty tricky variant of Scheurle porting, but with a much simpler crankcase casting.