I certainly can appreciate your point that a true MC10 was a sub-10k rpm saw engine. I think that I muddied the waters by specifying that this rod is in a MC10 clone. Keep in mind, though, that this is the later style rod, which was used in McCulloch's "racing engines" through the end of production.
I get it though... The rods, as they were, were "good enough", and at a reasonable cost. It's been my impression that by keeping the costs relatively low, and creating an economy of scale, McCulloch was able to become standard karting equipment. Just like it's cheaper to race a Chevelle, when compared to fielding an AMX.
I had only opened the first pic of your con rod. I did not see the piston you have there. It seems to be an aftermarket stroker piston for the 1.625 crank.
Something else to note: the top of piston is scalloped to open the intake ports sooner.
Below are stock chainsaw pistons. Left is for 1.375" stroke.(Mc5) Right is for 1.5" stroke.(Mc6/10) The skirt is slightly longer for the bigger stroke. Your piston is hugely longer in comparison.
Hi Doug,
Yes "free-porting" means that the bottom of the piston skirt opens the exhaust ports into the crankcase instead of being long enough to keep the exhaust ports closed as the piston reaches the top of the cylinder. In chain saws and at the RPM"s they are running and with open headers or mufflers, it will be acceptable, though NOT the best, performance wise.
In a "high reving" modified Kart engine, free-porting causes a breaking of the standing waves through thru the carb/s into the crankcase that happen above certain RPMs. When this is happening, the reeds NEVER close, but stand open ALL the time. The engine will run with a reed GONE and there is NO sign that a reed has broken and is gone. The engine starts to act up only when the RPMs drop below the point that the standing waves form. If one can keep the engine running, and slowly get it back up in the RPM range it will start running perfectly again. Steve Ohara learned this when he lost a reed while racing, and has talked about this in posts. He re-learned something that had become lost knowledge that "old timers" knew about, but they had stopped racing so it had become lost knowledge
AND NO, NO, NO free-porting a "piped" engine is sure DEATH for the engine!! For the pipe to work right, the engine MUST NOT free-port. Pipes use BOTH sound waves and pressure waves to work their magic. Free-porting would destroy how they work.
Hi Terry.
You are correct about the pistons. The shotest piston was used in saws with 1.375 stroke and the Mc-5 Kart engines, also 1.375 stroke. The longer piston was used in saws with 1.50 stroke and 1st year Mc-10 kart engines (all had thick rings) also 1.50 stroke. The skirt legnth of the piston was increased so the exhaust did not free-port. ALL Mc-6 pistons had THIN rings, but the piston was the same except for being thin ringed, as the one you showed. Late model Mc-10s had the thin ring piston used in Mc-6s. ALL Mc-10 stroker (1.625 stroke) pistons had thin rings. The Mc-10 stroker pistons had a longer skirt legnth than the longest piston you show (being 0.080 longer than the pistons used in saws with a 1.50 stroke or in Mc-6s/Mc-10s) again so the exhaust did not free-port. The piston on the Mc-9 rod that Kirk showed was, in fact a stroker piston for 1.625 stroked Mc-6/Mc-10 engines. The top ring is also 0.0625 lower so it doesn't come out of the cylinder at TDC. 1.625 stroke mc-6s/Mc-10s have the piston pop out of the cylinder at TDC
between 0.0625 and 0.065 thou. A THICK head gasket is used so the piston does not hit the head. The longest skirt piston creates no problem for clearence on the bottom of the stroke. And YES the scolloped piston top changer the intake port timing by opening the intakes sooner. It is probably a Moss Piston.
One thing to note, that I realized while speaking with Torque the other day, is that the third point timing is moot on this engine. The intake manifold is of a MC45 / pyramid reed design, and effectively deletes the third port effect. Also, the cut-away in the skirt matches the skirt length of a standard MC49 piston. Here are some pics of those, side by side:
More pics. It's a long story, but long story short, I've found myself stroking a S44 block from 1.375 to 1.50, with an S44A crank. It's been a fascinating learning experience. Here's a couple pics showing how each piston stacks up:
04-11-2020, 11:58 PM (This post was last modified: 04-12-2020, 04:19 PM by Terry Bentley.)
There is a height difference of about 0.060" between the D44/55/Super 44/55 and S44A/S55A blocks. By changing out the crank, piston and much thicker head gasket, you convert the Super44 into a model S44A. The standard style blocks starting with a D30 have a higher deck height than D44/S44 of about 0.030". The Mc10 style blocks, 1.500" stroke, S44A/55A, 1-70/80, 1-71/81 and 1-72/82 all have the same deck height that is another 0.030" taller.
So basically two different deck heights between the standard and super 4.9 ci motors. Super models the pistons pops out about 0.020". Standard models piston sits down in the hole about 0.010". All of the 5.3 ci motors with removable heads have the same deck height, which is about 0.030" higher than standard models. Have I confused everyone yet?
(04-11-2020, 08:36 PM)Alvin A Klusman Jr (Torque) Wrote: AND NO, NO, NO free-porting a "piped" engine is sure DEATH for the engine!! For the pipe to work right, the engine MUST NOT free-port. Pipes use BOTH sound waves and pressure waves to work their magic. Free-porting would destroy how they work.
Best Regards TORQUE
Sound waves are pressure waves. I think you meant "and resonances". Resonances being the frequency that a specific volume of gasses wants to resonate at a certain set of conditions like at a given temperature and pressure. Like the natural frequency of a whistle or organ pipe. Those conditions (operating conditions) are very dynamic in a 2-cycle so there in is the challenge.
