Displacement at .032 Over?

[Dennis Detweiler]

What's the displacement on a Mc 91 at .032 over?

[Tom Davis]

Dennis,
6.1983 cu. in. or as some prefer 101.5705 cc. On a new set of rings and a cold block that will be subect to change in about 150 yards.
Tom

[jmendoza]

As you can see, boring adds very little disslacement to a MC-91 series. The biggest advantage is a fresh bore and rings that have the potential to make power once properly seated/broken in. It is not uncommon to realize a power increase of 20% when an engine is porperly rebuilt and set-up. This is not 20% over stock, just 20% more that before, especially if it was worn out and out of tune. Blue printing also gives noticable increases in power, and prolongs engine life.

[Jeff Campbell]

... many have bored their 91 series blocks to accept the MC101 piston (2.280 std bore), that gives ya about 11cc more over the stock std bore 91 series motor. Then you have to deal with re-balancing the crank to accomodate that big heavy piston (weld on MC101 type crank counter weights), which will really stress your rod even with the crank balanced ... good luck.

You'll have a more reliable setup, focusing on making a stock motor run well, along with setting up the pipe and clutch correctly.

OK, I have to run and finish building my modified Nitro-Alky MC93 for East Lansing this weekend, LOL! Watchout for the orange Margay Cheetah.

Jeff

[VirgLove]

I read somewhere once that rings just provide the compression needed to start the engine and once its running it probably doesn't even need rings. Now I can't remember if this was for Foreigners with a half a thousandth piston clearence or Macs with enough piston clearence to hear the piston slap but I thought it was an interesting theory especially considering all the time we spent setting end gaps and breaking in engines to seat the rings.

[Rob Voska]

Good seal on rings also keeps the heat in the piston crown (where it's cooled by the incoming mixture on top and from the underside) and not down the side of the piston heating and expanding the piston and burning off all the oil.

[Jeff Campbell]

I think Rob has it right there, Jordon Jennings also wrote about that in his book "The 2-Stroke Tuners Handbook". Jennings mentioned how ring flutter at high rpm would lead to thermal melt down (siezures, holed pistons, etc...) in a racing engine, from additional heating of the piston caused by hot burning gases shooting down the side of the piston.

Jeff