Mac 1-50 MC5 conversion

[Tom Smith]

3 top transfer ports, 3 bottom transfer ports, and 3 piston ports = 9 ports (9 ported)
later macs with 2 boost ports = 8 ports (8 ported)
early macs with the 3rd piston ports blended into 1 larger single port like an Mc-90 and early 101 = 7 ports (7 ported) Some sharp dude must have figured out all that higher math, LOL.
The way I understand it the McCulloch 3rd port or piston port which ingests fuel that is bypassed by the reeds main purpose was to aid starting and idling, primarily for the chainsaw powerheads which all the early kart engines were derived from.
As far as a box muffler being period correct on an Mc-5 goes, I'd say an early style open header such as a Palmini would be far more period correct.
The tapered cranks from a model 380, 380A and 440 gear drive saw are the same stroke as the Mc-6, however they use a PTO ball bearing instead of a caged needle roller bearing. It might be possible to modify the bearing perch area to suit the Mc-5/6 style bearing.

[Rob Voska]

Just my two cents worth. It's a $20 saw block so no big deal if you trash it. Study, measure twice and cut away as you desire. Sure you will make mistakes..... but the education will be more than worth it. I would also suggest that you run it first so you have a baseline on your modifications. Have fun & cut away.

[Jeff Campbell]

Rob,
Right on the money, after all that is what saw blocks are for, LOL! ...one of the reasons I built my MC5 look-a-like motor was to have some fun doing some porting work, I get nervous about cutting away at a real kart block... since I may make a mistake on a valuable block and because I want to preserve the past (not consume it). Of course some of the vintage saw guys (McBob), have a completely different perspective.

Tom,
Yes, I think that is the only way to count those port numbers. I always wondered if the exhaust ports were counted too. To add the my confusion, Wiseco continued selling 9-port pistons for the Macs well into the MC91 era when 91's had one large boost port or the two finger boost ports (not 3 boost ports as on the MC90). The catalog page portion below shows their "9-port" piston and their legal stock class piston (implied not a 9-port piston):


...and then to cause complete confusion, Wiseco had their super duper forged MC91 9-Port piston that clearly only fed 2 boost ports (hence an 8-port motor):


I guess it's Wiseco's fault that I never understood the "9-Port" thing.

Jeff

[michael kent]

Now it was mentioned to cut out the missing transfer through the welsh plug like I assumed the factory did but the last one not drilled out does not have a plug. Do I go through the block? I would feel much better about leaving it be as is unless it makes a huge difference in performance. I Did run the engine as it was a saw and the thing had what seemed to be ALOT of power. Although, all things considered, my only other saw is a 30cc piece of garbage.

[Scott Kneisel]

I use a 1/2" spot facer to make the first pass at the blank port from the outside of the block. The spot facer will mill the 1/2" indent for the welsh plug while providing a guide hole for the port. Once the indent is milled just deep enough for the plug, I then drill out the port with the correct size drill, then install the plug.

I have successfully done a few saw blocks this way but have also runied a few along the way. Getting the compound drill angle correct to maintain the port spacing and height is tricky at best.

Scott

Spot Face 002.jpg

(266.37 KiB) Not downloaded yet

Spot Face 001.jpg

(298.32 KiB) Not downloaded yet

[Jeff Campbell]

Mike,
I guess it depends on how much you want to bit off and do. I usually find that once I put one of these engines together, we don't get enough run time on them to justify tearing the motor down again anytime soon to do additional work. The macs do tend to have too much transfer port area anyway ... if you boost port it, the 2 transfers on each side might be ok ... certainly raise, square and thin ribs on the exhaust port. You do have to have 2 transfers on each side, if you already drilled the 3rd on the easy side, you are now committed to doing the other side (it must be balanced).

Jeff

[michael kent]

Mike,
I guess it depends on how much you want to bit off and do. I usually find that once I put one of these engines together, we don't get enough run time on them to justify tearing the motor down again anytime soon to do additional work. The macs do tend to have too much transfer port area anyway ... if you boost port it, the 2 transfers on each side might be ok ... certainly raise, square and thin ribs on the exhaust port. You do have to have 2 transfers on each side, if you already drilled the 3rd on the easy side, you are now committed to doing the other side (it must be balanced).

Jeff

Dang I didnt realize about balance. I already drilled the 3rd port out on the manifold side. It was easy because of the side indent I was able to get the ange correct. What I Did was start with a small bit then made sure it was centered then progressively went bigger to eventually 7/16" making sure the angle and spacing was correct between the ex and in ports the whole way. I still have some smoothing to do on the edges and was going to champfer the edges slightly for good ring life.

[michael kent]

Oh one more quick thing, How much should I raise the ex ports before being squared? Now From what ive seen it appears the inside nearest the ribs will get squared off and the outside port edge is left rounded ? From what ive read a round port is much easier on rings than a square port.

[Jeff Campbell]

Mike,
For what you are doing, I would not go crazy with raising the exhaust port ... I would suggest keeping it tame at 0.020 to 0.040". The exhaust muffler, has to be matched to the port. You can angle the port upward as you raise it so you don't have to hack away at your muffler too much.

When you thin the exhaust ribs don't go too thin, doing so can result in thermal issues, the thinner ribs will run hotter and warp. No need to go thinner on the ribs than 0.060", shoot for 0.080".

Also, when you "square" the exhaust port tops (don't bother with the bottoms), you really shouldn't make them a super sharp square corner, you are basically putting a small radius there (the size of a small round file or one of those 1/8" dremel bits), and like you said, you should not square the outside corners of the exhaust port.

Jeff

[ted johnson]

McMaster Carr has a carbide granule 60 grit 1/4" X 1/8" file, P/N 81725A11, that makes short work of roughing the exhaust ports. It doesn't have a sharp corner to it. Smooth them and do the final corner radius with a chain saw file, 3/16" or 7/32" diameter. The chain saw file has a constant diameter and cuts both the aluminum and the iron liner very nicely and smoothly. Do the final finish with 180 - 220 grit wrapped around the saw file. If you're running gas, the nice sanded finish will help limit carbon buildup. Using the carbide file saved me a LOT of time on the Mc5 I just did. TJ