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I am looking for education. I was under the impression that when you 9 ported the early Macs you put a "9 Port" piston in the engine. I thought the 9 port pistons had the window or cutout along the piston wall like the 91 and later pistons. The third port engines through the Mc9 had a full skirt.

I am working on a Mc 90 which is factory 9 ported yet the piston is a full skirt (like the Mc9 piston, except for the rings being pinned). Do you get full advantage of the porting or did McCulloch not complete the job when they built the Mc90? I have a NOS Wiseco Piston that says it is for the Mc 8,9, 90,& 45 and it too has the full skirt. I am confused.

Bob,

The 45,90,75 and 100 models came with the factory "9 ports" added and they do not need a "9 port" piston because they removed the partition between the intake area and the third port passage so the boost ports are fed through the third port channel.
The modification means that those motors cannot be run with any of the flat reed plate manifolds or with the Mc 30 and Mc 40 combo reed cage/piston valve designs.
For the 45/75 they eliminated the flat reed plate used the old basic saw manifold and replaced it with a Go Power pyramid reed cage. The hole under the carb had to be bored out to fit the reed cage and the studs were lengthened. For the 90/100s they junked the saw manifold and upgraded to the Reed sloper manifold still using the Go Power six petal cage.
With the intoduction of the 91 and 101 models the blocks were redesigned and the third port passage was eliminated. The 91s were fitted with the first of the "9 port" pistons with the "window" or cutaway area to allow the boost port to be fed up the side of the piston. The first generation 101s were the real odd birds as they were fitted with pistons that had no window or cutouts to feed the big square boost port. At first glance it is easy to question the IQ of the engineers until one looks closer and realizes that the first generation 101 pistons were the first to have the blind bearing on the exhaust side. Since the bearing on the exhaust side was no longer exposed to the port the bearing no longer needed to be a closed end bearing as they had been in previous models so the boost port was fed through the wrist pin. Feeding through the wrist pin is good for cooling the bearings and pin but it restricted the flow so it did not take long for Wiseco to produce a piston for the first generation 101 with the full window on the boost port side to take full advantage of the extra port area.
Hope this helps!
Steve O'Hara

Steve.....

information is sometimes trouble....(to me).....why is is better to fill the "9 ports"" on the older motors with epoxy....and then use a newer 91 style piston,,,,,???? or...is better?????
thx
gary

Steve,
Thanks as always for a thorough explanation. It makes sense now.
Bob

Gary,
Filling the 3rd port channel and installing a 91 type piston serves to stuff the crankcase a little bit so it is configured more like the Mc91 and up. I have tried it both ways and really can't see a difference. The only thing I have observed is that with the 45-100 configuration 9 ports with the wall cut away my engines seem to run better with the Mc45 type manifold than with the reed sloper. The Mc45 mani puts the reed pyramid up the cylinder a bit and I think it feeds the boost ports better when using a full skirt piston. If you use a 91 piston it wouldn't really make any difference. I have only observed this on my 49's which can't use a 91 piston anyway so I have to feed the boost ports thru the opened up 3rd port channel.
Now I have probably confused you more because I am becoming confused and I am writing this...LOL!!
What the hack was I talking about??? CRS
Later,
Scott

Scott....

thanks much...I've had guys give me reasons to do it both ways.....and actually I do have 45s filled and a 90 not ......but I can understand the ""advantages"' of filling them,,,I think...any how, thanks much for trying to clear my mind up.....time for another drink...LOL...maybe that will help.....

Happy New Year...

GW

With a 9 port piston and the passage filled in the flow is restricted compared with the factory setup as they used in the 45-100 models. The bottom of the boost ports is nearly covered by the ring lands when the piston is at or near BDC so there is only a very short period where the areas above and below the ring lands match up to allow much flow. With the original design the flow to the ports is unrestricted and the capacity to move charge grows as the piston moves down from the point where the tops of the boost ports are uncovered.
I don't think it makes much difference if a simple box header is used but I would choose to give up the stuffing benefit and keep the better flow if I was building the motor to use a pipe.
Steve O'Hara

Steve....

thanks much,,,that clears thing up.....

gw

Just a little thought...
When the Rookie Class ran the 49C (flatback carb)..
We found that if you blocked off the 9-port passage, the motor was faster.
We would use an block to Reed plate gasket that blocked off the passage.
In addition, we use to make a plug to "stuff" the passage.
I have used 9 port pistons, in early engines, and they ran great..
Just my 2 cents worth...
Rick

early 9 port piston ad with some Tom Medley artwork.