early Mac (Mc5) parts

[Carl Haydt]

Mc5 pic

[steveohara]

The specs show the Mc 5 has a stroke of 1.375" and the Mc 6 has a stroke of 1.5".... this raises the question... how did they use a common block with two different stroke cranks? Did they use a shorter rod or piston with a different pin to top dimension? The piston would stick up an extra 1/16" and that would be too much to correct with a head gasket change.
Anyone.... Tom??
Steve O'Hara

[ted johnson]

Steve, I'm old, and my memory isn't what it was (and it was never much...). It seems that the Mac 5 piston actually didn't even come to the top of the bore. We were running gas in those days, and if memory serves, we ran about a .010 gasket (can't remember what the stock gasket was). If you checked your engine with Plastigage, you might find that you could lap the head and run without a gasket. The Mac 6 conversion came with, I believe, a .066 gasket as the piston popped out of the block about .031 with the 1/8" stroker crank. That appears to indicate that the pin height in the thick Vs. thin ring piston was the same...? I remember a few guys running stock stroke (1.375) '5s with Mc10 carbs, thin head gaskets and thin ring pistons, and they perked right up and ran quite well. TJ

[david a luciani]

steve,
i'd be guessing about the stroke but what ted says sounds right.
when mcculloch stroked the mc10they included the thicker head gasket.
it's also possible they used the mc10 piston since the pin is higher to the deck.
when they built the mc20 they did raise the block hieght and change the pin height on the piston too.
many of the older saw blocks have pistons that don't come even with the top of the block as was speculated about the mc5.

from direct observation mcculloch was in the habit of restamping blocks and mix and matching parts.
i've pulled down lots of saws at this point and the mixing of parts seems to have been routine.
in fact almost 1/2 the old saws are restamped with crossed older model numbers.
i even got a kart block from a bone stock saw that had undisturbed paint.

three of my all original mc10's are restamps with the saw number crossed out.
i was told all the mccullochs were basically bench built motors assembled by hand.
in that kind of factory setting the assembly of the motors would be a bit more casual .
anyone dealing with a nos motor would know if they were to go through the motor preping it to run..

if there was a shortage of blocks out would come the stamp and presto the 1-70 is now a mc10.
the stamping of the six was probably a bit like that.
some guys would maybe restamp others would add the slash and six and say good enough.

i have another mystery motor.
it's a super series motor but it's stamped mc90d.
i'd always thought that the mc90's were the regular style block with the longer crank.
i really hadn't looked at it till recently but on close observation i can see the exhaust port had the ribs removed and the new oval port was raised.
the strange part is that the middle port had been solid as you'd find in a saw block.
next time i'm at the shop with my camera i'll get pix of that too.
dave

[steveohara]

Guys,

The Mc5 I have here has a .065 head gasket and the piston comes up flush with the top of the block. The longer stroke would push the top ring on the piston right up to the very top of the cylinder so that does not sound like a configuration the factory would have chosen. The shorter pin to crown piston seems more likely.
Thanks for the info.
Steve O'Hara

[ted johnson]

Just for kicks, Ron C. bumped an Mc5 piston against an Mc6 piston. Seems the pin height is the same. We're thinking they just juggled head gaskets to get the clearance. Also, we need to remember that the factory compression ratio on the '5 is listed as 6.5/1, whereas the '6 near as I can tell, is around 8.4 or 8.5/1. Mac even released .080 head gaskets in case the .065 gave too high a ratio when converting the '5 to a '6. We're going to use a thin gasket and the Mc6 head with the small "golf ball" on my engine, along with a later alky Tilly and the Hovey manifold.
B.T.W. I painted an old XP Homelite shroud with Rustoleum light gray primer and Sunburst Yellow rattle can paint. After it dried, I bumped it against the powdercoated shrouds on my 101/Mc20 "semi clone". They're VERY VERY close to identical, even in the sun. The powder color is RAL 1023. I feel safe in using the Sunburst Yellow on block, flywheel and head, and the RAL 1023 powdercoat on the shrouds for strength. This has been a fun thread, thanks, everyone! TJ

[david a luciani]

ok i was bugged by where and how i knew that mcculloch used the shorter pin to deck height piston in the mc6.
i knew i'd been told that but i'd also read it somewhere so i went looking.
got lucky and found a reference quick.
it's in my book "Kart Engine Manual by Jim Butler ' it's a compolition book of karting world engine articles mostly.
Anyway on page 93 it says this:
"on the inside
The mc-6 kart engine is very similar to the mc-10 engine and this similarity is more than just skin deep.the same piston and rod assembly is used in both engines and the cylinder cases, with minor exceptions, are identical.We will note the important differences between these two engines as we come to them."

when i first cross referenced the mcculloch ipl's i found that the pistons weren't listed as the same one on the mc10 and mc6.
i then checked the mc5 and the rings on the mc5 55123 were the same as the rings on the mc10 piston.
since the mc5 only had fat rings ie: chainsaw version i assumed the piston and rings listed in the ipl were the earlier mc10 thick ring pistons.

then i checked the mc10 hop ipl and while the piston doesn't have a number the rings do.
it's 48691 the same as the mc6.
then finally on page116 of the engine manual i found a picture reference to the mc10 hop up piston comparing it to the regular mc10 piston.

here's the picture with the text:

page.JPG

(279.74 KiB) Not downloaded yet

as you can see both the later stock piston and the hop up piston have the same thin ring.

so in my twisted way near as i can figure the mc6 had the same piston as the mc10 hop up piston and the mc10 regular pston had the same thin ring as pictured.
so mcculloch must have used that piston on the mc6 also to allow clearance.
as usual the mcculloch ipl's aren't exactly accurate or weren't updated all the time.
it would have been alot easier if i could find my updated list of mcculloch pistons but i'm bad with paperwork.

also i think Scott built a mc6 using a mc10 piston as i recall but Scott probably will have a better recall of that then me.
dave

[steveohara]

Nice work Dave!

If I was really ambitious I would take apart my Mc 6 and compare the parts to my Mc 6.... but I'm not
Steve O'Hara

[ted johnson]

There's no reason that a thin-ring piston for a stock 1.5" stroke Mc10 couldn't be the same as an Mc6 piston (same 1.5" stroke), though I never saw or heard of a thin-ring short stroke Mc10 from the factory. The stroker '10 and Mc6 rings would be identical. The stroker Mc10 piston had the rings moved down to prevent the top ring popping out of the bore, due to the .135" (I think that's the right number) longer stroke than the unstroked '10 and the '6 The Mc6 rings were higher on the piston, at least the ones we had. There was no reason to have the same piston as the stroker '10, as the Mc6 came out earlier than the stroker. We used to hand scallop all our pistons, and the scallops on the stroked '10 pistons could be deeper than on the '6, due to the greater top ring land thickness. Scalloping on the '6 piston had to be shallow so as not to get too close to the top ring. Dad and I built several Mc6's and even more stroker '10's, so that part I DO remember, though the memory ain't what it used to be ! TJ

[david a luciani]

ted,
the scalloping makes total sense since the mc10 stroker piston does have lower ring locations allowing a bigger scallop.
you have opened a big can of worms though by mentioning knowledge of scalloping pistons.
that's something i have little info on and would love to have more so i'll be bugging you about that at TBO.

the easiest way to confirm what the karting world people said ie the mc10 and mc6 piston are the same would be to get the numbers used by mcculloch for the thin ring piston on the mc10 and compare with mc6.
or having a nos mc6 piston and a nos mc10 piston and miking them up.
i haven't got that info anywhere i have looked but i will continue that search.

i know the mc10 had a different piston from the mc20 at least.
on a mc72 block i have we were gonna build a mc20 clone.
it's stroke is mc10 with the mc20 points set up and bearing.
seemed like a mc20 could be built from it by changing the crank.
we discovered a problem when a mc20 piston was installed and cleared the ring land at the top of the stroke.
we figured out the mc20 deck was raised a bit from the mc10 block.

later i had a mc10 piston that fit and a mc20 crank and for giggles put them together in the block.
the piston fits and the ring land stays below the top of the bore.
i haven't gotten around to actually building the motor (to much else going on) but it may work that way.
of course there may be timing issues too.
i kinda doubt it because it is the correct piston for the block.
the other piston was a mc20 and the right oversize and a mc20 was being built soooo.
the point of all this is the stock mc10 piston does have a different deck height from the other later pistons and can possibly be used when stroking a older motor especially clones from saws.

anyway a great thread and soonish i'll give you a call about those manifolds.
dave