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Can anyone tell me the main differences between a Mac 8 and a Mac 9. Also will parts from later models fit either of these two i.e crankshafts, rods etc.
Don Tomich

Don,
The 8 and 9 are very close to identical if you just look at the parts and not the part #s. I think the 9 uses a 5 degree flywheel and has smaller keys in the crank at each end. The cranks, rods, bearings, pistons, heads, stuffers, ignition parts, most gaskets and fasteners are all interchangable between the 7, 8, 9, 90, 91, 91A, 91B, 91B1. That does not mean they are the same, just that the parts from one model will fit the others. There are a few exceptions... post a specific question if you aren't sure.
Steve O'Hara

steveohara wrote:Don,
The 8 and 9 are very close to identical if you just look at the parts and not the part #s. I think the 9 uses a 5 degree flywheel and has smaller keys in the crank at each end. The cranks, rods, bearings, pistons, heads, stuffers, ignition parts, most gaskets and fasteners are all interchangable between the 7, 8, 9, 90, 91, 91A, 91B, 91B1. That does not mean they are the same, just that the parts from one model will fit the others. There are a few exceptions... post a specific question if you aren't sure.
Steve O'Hara

Thank you for that!

I recently got a nice MC9 (in pieces) and there were no connecting rod needle rollers. So can I assume that anything available for the 90-91 Series will fit? Naturally, I would scrutinize the clearance once assembled.

Randy,

Yes, the big end of the rod needles are the same for all the Mac kart motors. You'll find slight variations in the shape of the tips on some aftermarket needles but the overall length and diameter of the needles are all the same.
Point of interest..... I have seen a Mac 9 with a large ball bearing on the PTO side of the crank just like the Mac45 uses..... I don't know if it was a variation from the factory or something someone modified. I can't rememeber if it used the larger diameter seal like the 45 or still had the small seal.
Steve

Karting World from the M10/6 days showed that mod, changed the 6 to the dual bearing machine work, but since I don't have the mag with me can't tell about the seal. The pto inner side was opened up for the sealed ball bearing so if the bearing fit the 6 crank dia od the seal might have remained stock. By the way, anyone have a good M6 crank they'd sell or possibly trade for ?? TL

Racing Karts, September '61 had an article on putting Mc10 bearings in your Mc6. They used the Mc10 seal in the conversion, boring the block to fit. The Mac 10 crank was turned to .6694 Dia, and they used a .002 lead-in on the journal for the bearing bore to begin its fit. A cleanup tool was used to ensure the 10 bearing went all the way onto the crank. I imagine that the later blocks would take the ball bearing just as well. Ted

Thanks Steve.

Thanks all for the information. Most have my questions have now been answered.
Regards,
Don

MC-9's used two styles of crankshafts, the difference was in the keyway length and location. Crankshaft p/n 48586B requires a 1/2" key, crankshaft p/n 48586C requires a 3/8" key and the flywheel keyway is retarded 3 degrees clockwise (viewed from the flywheel end) when compared to crankshaft p/n 48586B.
Mcculloch advised their dealers that because of the difference in location of the flywheel keyway on these two crankshafts, it was especially important that a degree wheel be used when setting the breaker point timing on the MC-9 engines.