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Hello, I am wondering if anyone knows of a modification that puts a ball bearing on the PTO side where there should be a needle bearing. According to Dave it was to, in theroy, make a more balenced motor. Just wondering what other people think about this.
Brian

Brian, I have seen blocks with this modification, so it is definetly possible. My thought is that it is not worth the work. As to performance, I have never had a 92, 91mc, or 93 that ran as good as a 91B or B1. There are possibly other reasons relating to porting in those cylinders that probably account for the difference. In theory, two ball bearings would be better, if and a big If, the crank alignment is correctly machined. Also, the fit of the bearing to the bearing pocket is very critical, and would have to be very closely watched. With the brass sleeve in a 91 style engine, this almost never comes into question. So, after all of that.....make your own conclusion, either way will most likely work fine. Best of luck, Sonny

Sonny is right... the motors run batter with the standard 91 type crank and caged needle bearing.
The onely reason you would want to bore out the block to fit a ball bearing is if you don't have the correct crank and if that is the case post here and see if you can trade for the right crank.
Steve O'Hara

Actualy when I bought my Mc 6 motor, off ebay, it was already modifyed. I was just looking to see if anyone knew why they would to this.
Brian

A number of Mc6's were modified in "the day" by adding a ball bearing to the PTO. One of the big name engine men, maybe Jim Ellison, did an article, maybe it was in "Racing Karts" or "Karting Sport" magazine about putting an Mci0 bearing in your Mc6. They ballyhooed it up as being stronger and more reliable. Both true, but NOT faster! One of the reasons was that a number of 75 and 100 mile enduros were being run, usually through VERY rough Mexican border town streets. I do know that the article writer was big into this. TJ

Brian,

quit worrying about that bearing it is what it is.
your modified block is a kinda a rare mod but the engine will run fine i'll bet.
PLUS you can't really change it back to stock without spending a stupid amount of money.
i say put it together carefully and run it till it blows up or you get a bigger motor.
otherwise next time i see you it'll still be apart and i'll be ahead of you again!!!!
dave

This was a very popular modification. I have ran across 3 engines with this mod, One of them was a stock port NOS that had been modified buy the delear prior to sale. Remember this was being done in 1961 and they did not know the stock configuration was faster yet, there was no 91b's, the only comparison was to Mc5s and 10s. We as modern day vintage karters have a real adantage to compare things from over 50 years of history, these guys were just trying to figure out what works with very little background. Who knows maybe Mc6s were faster with this mod, I dont think we will figure that out being we are not racing all the time....Just have fun building and running it. ron

Thanks for all the responces. Dave i am leaving it be for just those reasons also the purpous for asking this was just to see what this modifycation was for and thought to do back then.

no worries.
give rick a call for the rings .
let me know how you're progressing too.
dave

Brian,

I agree with the previous posts... it was a common mod back in the day. I have a Mc 8 or 9 that has been set up with the big PTO ball bearing. The performance difference is hair splitting so the guys are right... leave it as it is and enjoy it. One factor that may have been a contributor is that the early motors like the 5 and 6 models had a full complement roller bearing and the later models 90, 91 etc went to a caged roller bearing that keeps the rollers from working against each other. There may have been reliability issues with the full compliment version of the bearing that we just don't have with the caged version. We all have seen how the full complement needle bearings in the piston have often failed so it may be there was a good reason the folks did that modification.
If it were a decision to make today based on what we know of the reliability of the later type bearings there would be no reason to do all that machine work.
Steve O'Hara