Finger Port Depth ?

[Bill Hermsted]

First, happy New Year!
How deep do these finger (boost) ports have to be, that some of you are adding, before they really do some good? I don't want to get too eager with the stone, as the PP and early Mac cylinder lack those nice bumps on the outside to allow room for the nice factory fingers.
bill

[Sonny Gerber]

Bill, I would copy the depth of a 91b-b1 boost port. Also, it's a good idea to build up the outside of the block with Marine Tex epoxy (it's fuel resistant). This adds a little strength and insurance in case you cut too deep. It should take no more than a 1/4-3/8" depth of coverage. Much more than that and you risk the coil not fitting (from experience). Good luck.

[Scott Kneisel]

Hey Sonny,
I measured the boost port depth on my Mc90 block. They are 0.200" at the deepest point right above the 3rd port opening and taper to flush over the length of the port. These were factory done with a mill from the top as was a common method of modifying the older blocks.
Attached is a pic thru the exhaust port.

Have fun, don't nick the bore!!!! Been there done that.....

Scott

Mc90 Boost Ports.jpg

(113.11 KiB) Not downloaded yet

[Terry D Bentley]

You dont have much room on the Power Products cylinders. What I did on these was to weld a cover over the added ports and feeders to them. You can run shallow fingers up the bore instead. Just dont run them all the way down the liner. On depth, go until you hit aluminum and will work fine. Make sure holes drilled in piston are high enough to feed at BDC. if you want to ensure enough area, just slightly teardrop the fingers and it will work fine. I may still have a cylinder laying around with the ports added somewhere.

The older Macs do have a whole open passage behind the third transfers to feed the newly added fingers. On the Mc 90 fingers are running about a 22 degree angle. This angle is not totally critical if you angle the two outer boosters in towards the center. If you are adding them to an old chainsaw block without third transfers, two larger fingers can be plunged in from top to be just like the 91B and newer ports. I set a rotary table up on the mill, indexing the cylinder to add the missing intakes and add boosters on chainsaw blocks.

[Bill Hermsted]

Scott- Great close- up!
I see where the tops of the fingers are slightly lower than the tops of the transfer ports. Is this what you shoot for?
bill

[Terry D Bentley]

I have a Mc 90 block that has never been mollested. The fingers are almost same height as intakes. Not nearly low as ones in pic. They dont need to open before the intakes. With the tolorences in production, slightly lower would yield fewer scrap parts. These boosters must have been a last minute decision in production. The third transfers had already been drilled, just like in prior models. But the exhaust ports, (port in this case) had to be finished before the third transfers could even be added. It would have saved a tooling setup if just two large or three smaller ports were plunged in from the top instead of added after the third transfers were drilled.

[Jeff Campbell]

For reference, I have McCulloch toleranced drawings for some of the MC90 and MC91 parts (this is the MC91 with the boost port cover, single large square boost port cut in cylinder) ... the boost port height and transfer port heights are exactly the same on these 2 motors relative to the top of the cylinder. The boost port height (port top to cylinder top) tolerance range is: 1.273 to 1.303", and the transfers are: 1.301 to 1.321". So, in this case the boost ports should be about the same height or about 0.050" above the transfers. On post MC91 series motors those 2 finger boost ports were below the transfers, but most of the modified blocks I have seen, usually have the boost ports raised to the same height as the transfers.

In case you want to see more on these drawings .... here is a link to a document I set up to view them...


Jeff

[Bill Hermsted]

Last question guys, promise-
Are the transfer holes in the piston drilled so their centerlines (through) are parallel to the wrist pin, or radially, ie- at right angles to the tangents at the piston skirt which lie over where the holes will be?
Hank fingers his PPs, and wants me to do one of my cylinders. Being only a four port engine, this mod must really help an AH 58 along. I'll start out with a stock piston, not the Moss nor the Fuller!
Thank all of you racers for the great advice!
bill

[Scott Kneisel]

Bill,
It must be the photo, this block has a standard bore and has never been modified. I put the piston in it the boost ports and intake ports open at the same time. There is tolerance though because I could see one middle back intake port sneeking open slightly before any of the others.

The boost ports in the Mc45, 90 and 100 were a result of independent modifiers doing the same thing on the 3rd port blocks. I guess McCulloch thought it was easier to just do some secondary operations on that side of the block because they did have to change the molds to accommodate the single oval exhaust port on those engines which started with the Mc40.

I have heard there has been some success in having the boost ports open first on Macs but I only tried that once on an engine for a karter that has not run it yet. I also have a 100cc Parilla in my shop that has been dyno tested at 30-34HP that has the boost port about 0.050"-0.100" higher than the intake ports(by eye). Maybe there is something to that.

For me, I will keep them the same height as the intakes.
Good luck,
Scott

[Terry D Bentley]

IAME tried the leading boost port on a run of the factory modified K-88's back in the early seventies. They did abandon the idea tho. Never heard any more about it. I remember selling one of those cylinders on ebay a few years ago.

Instead of drilling holes in a Power Products piston, just use a "6- ported" piston. They surely cant be that hard to find. I might still have some raw castings some place around here.