Understanding the 107in kits

Posted by By at 7 March, at 05 : 55 AM Print

Debunking the Harley Davidson 106/107in cylinder thickness and other myths…

I see and get quite a few questions with regards to how good these kits are and if they are worth doing. It just seems like the right thing to do, it’s no more expensive than a 103 kit and is a whooping 4 inches more… Naturally, in this area, more is always better. The entire idea of those kits to provide upgraded power at a reasonable price, and that they do. Properly done, such a kit can transform an ordinary Harley into a hell of a fun ride… but it is not at all trivial…

Reliability is another area where there is a lot of speculation with regards to the ability of the jugs (cylinders) to take such a large piston while keeping its integrity in the long run. One thing is obvious by now, with the number of those kits successfully installed and running on bikes out there, there is little question in my mind about the reliability of those properly set-up kits.

There is quite a debate with regards to the thickness of the cylinder walls of stock jugs when bored to 3.937 inches. I think its pretty funny to talk about the thickness of the jug’s sleeve when no one actually mentions that those kits are inexpensive because they are installed in the stock case. In fact, the reason for the easy (installs in a few hours…) installation is that you don’t have to go into the case. Those kits retain the stock spigot size! Now, if you don’t make the hole for the sleeve bigger in the case, you can’t fit a bigger sleeve in the jug. So unless you turn that sleeve down to fit the hole in the case, you just have to live with that spigot size/sleeve size. Naturally, you can make the heat sink (fins around the piston) thicker/stronger to a certain extent (the Axtell way…) or use no sleeve at all (the Millenium/Rev Perf way…) but you still live with the spigot size.


Now, when you are living on the edge like this, the edge better be even… that is, the bore centering should be spot on… which it usually is… Again there are so many of these kits out there for various Harley Davidson engines that you would end up with many failed engines if the set-up was not sound… no one would be selling them if they were that bad. What is my preference? My preference is for a stock cylinder that is bored to a piston size by a good engine builder/machine shop. I am not a fan of standard bore kits with standard pistons… when you buy a kit that is not bored specific to the pistons of THAT kit, you play the tolerance game… you can get lucky, but again, you can get unlucky… and that sucks. Mind you that usually you will get shafted on the loose side so you should be fine (remember that loose runs, tight does not…).

So I hope I have added something to the debate… my 2 cents worth…

Motorcycle Tech

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