mcculloch ringed pistons
Moderator: Rob Voska
- Scott Kneisel
- Posts: 477
- Joined: Mon Mar 24, 2008 7:48 pm
- Vintage Karting items owned: '63 Rupp Grand Prix, 66 Rupp Chappy, Mc6's, Mc8, Mc10's, Mc20's
Lotsa' other Macs - User Agreement: Yes
- Location: Farmington, NY
Re: mcculloch ringed pistons
Hey Steve,
The end of summer I had two engines go thru my shop that lost ring pins. One had a McCulloch Mc91 standard piston and they other a Wiseco 9 port 50 over in an 8 ported Mc8.
Reasons I think.....
The one with the 91 piston was really twistin' according to the owner, even had a little hot fuel in there along with the alky. Maybe just too much for the engine. Fortunately only had to go 5 over to fix that one.
The Mc8 had big ole "slot ports" right where the ring was pinned so I think the rings were probably catching a bit and worked the pin loose. On that one I had to go 70 over and I pinned the rings myself in a location where there were not any holes. So far I think that on is OK.
Those are my only experiences so far. Bill Mc told me that if the rings are not de-burred on the ends after fitting the end gap, that also can work a pin loose.
Lots going on at 12K RPM
The end of summer I had two engines go thru my shop that lost ring pins. One had a McCulloch Mc91 standard piston and they other a Wiseco 9 port 50 over in an 8 ported Mc8.
Reasons I think.....
The one with the 91 piston was really twistin' according to the owner, even had a little hot fuel in there along with the alky. Maybe just too much for the engine. Fortunately only had to go 5 over to fix that one.
The Mc8 had big ole "slot ports" right where the ring was pinned so I think the rings were probably catching a bit and worked the pin loose. On that one I had to go 70 over and I pinned the rings myself in a location where there were not any holes. So far I think that on is OK.
Those are my only experiences so far. Bill Mc told me that if the rings are not de-burred on the ends after fitting the end gap, that also can work a pin loose.
Lots going on at 12K RPM
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Re: mcculloch ringed pistons
nice to here from you scott ..hey got another one for you guys actually a couple first i have 3 or 4 mc 91s that have been needled pretty bad 3 i cant even get the piston out ive beat on it pretty hard i want the full circle crank out one ant the others all have pretty good lookin cranks couple are either polished or chrome cranks and rods ..but i cant get them apart it might come down to meting them out or busting them to pieces ..??? ..next question is there enough meat to bore a 91 so i can use a 101 piston .?? do they make a 125 over 91 piston ..i hate to throw them out .. thanks again guys ...
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Re: mcculloch ringed pistons
steve, good question on how big a 100cc mac can be bored and may actually complete a race. hopefully some interesting answers
Re: mcculloch ringed pistons
You can offset bore (front to back) a block that is needled but the liner will be a little thinner in that area. DO NOT melt them out. Just put muriatic acid down the edges of the cylinder. It might take a few days but will eat the aluminum away. I would do this slowly a little at a time. What do you have to loose they are already junk.
If all else fails call Ron Cubel. He's a pro at it..... but sometimes there is nothing left to find when
he is done. Right Ron.....

If all else fails call Ron Cubel. He's a pro at it..... but sometimes there is nothing left to find when
he is done. Right Ron.....












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Re: mcculloch ringed pistons
thanks rob ..i can offset bore a couple but thers a couple that have grooves all around not just front or back ..i used acid lotta times on my chrome bore motors ..has anyone ever done or tried to take a good liner out of a junk block and put it in a good block with junk liner .?? ..ive done it bunch of times on old tractors but thats kinda no compairison ..ive had new liners put in two stroke motors befor La sleeve did all my stuff back in the day .. go figure these blocks no chain rash ,broken fins ..bla ,bla ,bla .man to bad someone dosnt make liners for mccullochs .. ok one last question ..can i bore almost all the way threw the liner and have it plated few thou.. to get back to around a 50 or 60 over ... ??
- Scott Kneisel
- Posts: 477
- Joined: Mon Mar 24, 2008 7:48 pm
- Vintage Karting items owned: '63 Rupp Grand Prix, 66 Rupp Chappy, Mc6's, Mc8, Mc10's, Mc20's
Lotsa' other Macs - User Agreement: Yes
- Location: Farmington, NY
Re: mcculloch ringed pistons
Steve,
If you could use a 101 piston you would have to do some more things because the wrist pin offset is about 100 thou different. You might be able to just use a 101 rod which is longer or if you use a 91 block you could deck the block. Port timing would be affected more by decking the block, much like using a fast timed piston.
I have an Mc30 that is bored to fit the 101 piston. The wall is pretty thin but would probably work. I would use the block but the porting is awful and the thing probably wouldn't run worth beans anyway but I might give it a try anyway.
If the wall gets too thin you won't get good heat transfer from the piston wall and it might be really prone to sticking. Extra clearance might be a good idea.
My 2 cents.
Scott
If you could use a 101 piston you would have to do some more things because the wrist pin offset is about 100 thou different. You might be able to just use a 101 rod which is longer or if you use a 91 block you could deck the block. Port timing would be affected more by decking the block, much like using a fast timed piston.
I have an Mc30 that is bored to fit the 101 piston. The wall is pretty thin but would probably work. I would use the block but the porting is awful and the thing probably wouldn't run worth beans anyway but I might give it a try anyway.
If the wall gets too thin you won't get good heat transfer from the piston wall and it might be really prone to sticking. Extra clearance might be a good idea.
My 2 cents.
Scott
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Re: mcculloch ringed pistons
Lol .. ok now i dreamed up somethin new... punch out 91 to std 101 size.... machine for bearing run 101 crank and rod ..???? simple salution right ..?? 

Re: mcculloch ringed pistons
Sure if you want the rings to stick up out of the bore.
Liners are not smooth on the OD. They are cast into the block.
Liners are not smooth on the OD. They are cast into the block.
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Re: mcculloch ringed pistons
hhhmmm i kinda figured there was a catch .jeeeez im tryin to save the life of a few motors i guess ill take the easy way out ..throw them in a box put back on shelf and run my west bends ... 

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Re: mcculloch ringed pistons
ok guys .... like any other doc. i cant give up on tryin to save the life of a couple of these motors .. i spent mosta the day yesterday but i think i got one.... i found a chainsaw piston then bored it 2.217 .took care of all grooves .ground the rings thin .now i see they raised and eyebrowed the exhaust ..well whats the sense in that when its opened up bigger than the gasket or exhaust manifold they had on it ..just wondering what you guys do.. ?? dont look like much meat on a manifold or gasket to match them ..