on rebuilding mcculloch engines

[david a luciani]

Hi all,
for the last year i've hunted parts to built six mc 10's and three mc5/6 and 6's.
i am at the point in my engine projects where i need to start assembling them.
i've gathered up my main parts, sourced replacement bits and dis-assembled and cleaned the heads,blocks etc. that are needed.
now my problem.
i've looked at the ipl's till my eyes blurred, i've cross referenced everything but i can't decide on several basic things.

i have lots of the correct connecting rods for the mc10's and mc6's # 55009.
i've been told that these were weak and prone to breakage.
they also don't have an oil slot cut into the lower half as do the more recent rods.
these engines will be run and some have pretty extreme vintage port work done that would add stress to the bottom end.
So
1. can i replace these rods with the more recent ones?
2.if not can i at least machine the oil slot into the bottoms.
as i understand the newer rods were made narrower across the bolt line to allow a greater stroke and the slot is to allow more oil coverage to the bearing.but they all use the same bearings.so it would seem they would fit and work.

second question,
i've noticed that there were about four different piston bearings used in all the mcculloch engines.
is there any reason why i can't use a piston with the wrong part number so long as the bore size and matches the cylinder and the piston bearings are the same number as the correct piston?
thanx for the help, i'm anxiously waiting replies!!!
thanx dave

[steveohara]

David,

The rod from all the later model 100cc Mac engines will fit your Mac6. Also, many of the saw motors used the same rod and sometimes that is the cheapest way to get a good rod. A good rod for the 100cc motors often brings $75 or more on ebay but you can buy many of the old saws in good running condition for half or less. Many of the saw rods are the same dimensionally as the newer rods but they don't have the oil slot. For vintage karting uses the slot is not important... just run plenty of oil and they will run forever.
For the larger engines there are two completely different rods. The rod used in the 70,75,100and the first generation 101 with two exhaust ports is longer and has a larger hole for the wrist pin.... beware of some rods sold on ebay as "101" rods as they are the early version and can't be used unless matched up with the wrist pin and pistons from those early models.

Regarding pistons in general.... in addition to bore size you need to make sure the bearings are the correct size for the wrist pin, the pin to crown height is correct and in some cases the skirt has been shortened or notched to clear the crank. The Mac 6 is a smaller bore than the later models (7 through 93)so the bore has to be .040" over to fit a stock size piston from the later models. I don't know if the pin to crown height is the same.... someone who has more experience with the Mac 6 can let us know.
Regards,
Steve O'Hara

[david a luciani]

Thanx Steve,
i am now closer to building them.
using a bigger piston may be what i need to do.
Basically i think the mc6 was over bored but i'm not sure how much.
i had miked it but now i have some calibration doubts about the mike i was using.
when i miked a 93 i'm rebuilding it read the bore being 2.150 which as i understand is impossible because they were 2.165 stock bore.
i'm gonna borrow a better mike so i can be accurate in my numbers.
anyway, this info is gold to me as i have many motors that need rods and pistons and saw rods are easier to come by.
the wrist pin bearings on many of these motors are the same but i hadn't considered pin to crown hieght and while it should have been something i knew the skirt trimming i missed too.
many thanx again.
Dave
p.s. any more info is always welcomed
D