Diversion 600 cylinders mode for 51J, 3KM engines.

Some time ago I was asked on Yamaha XJ owners forum (xjbikes.com) about Diversion 600 cylinder block and pistons compatibility with XJ600 engines of previous generation, like 51J. I used Diversion cylinders in “Eight Ball” project, based on 51J engine, so even I forgot some insignificant features, but no matter  I had answer at hand. I replied on question in my thread on XJbikes,but let it also be here too.

So, As far as I remember Yamaha XJ600 Diversion pistons are useless for XJ600 51J because they have slightly lower dome height.  But cylinders could be installed with some modifications.

The advantage of using Diversion cylinder block on XJ600 (51J, 3KM) engine are:

  • Better cooling, as Diversion cylinder block has larger cooling fins.
  • Material of 51J and 3KM cylinder cooling fins is fragile, so many of cylinder blocks have broken fins. There is no such problem with Diversion 600 cylinder block.

There are few tips in this swap. All is OK with gaskets but Diversion cylinder block has another tilt angle then earlier XJ600 one, so when one plans to install Diversion cylinders into 51J or 3KM engine he has to solve the question not only of cam chain tensioner itself but also issue of not enough space around it.

That’s how all looked:

As you may see there are two problems with space:

  • There is not enough of it for original plug which hold rear cam chain dumper pin.
  • There is not enough of it to install tensioner itself and operate with it lower mount bolt.

So I grinded engine cases a bit, so I do with Diversion cylinder block and tensioner housing. I also screwed thread stud into lower tensioner mounting hole.

I also replaced original plug of pin which holds rear cam chain damper with custom made lower one (in fact I made it from brake banjo bolt) and shortened pin itself according to this custom plug.

Yet one modification caused by difference of XJ and Diversion cylinders tilt angles is rear cam chain damper. As tensioner now aim a bit lower I modified contact platform of damper.

And about tensioner itself. Original Diversion tensioner does not fit, it’s too long. I had no time to solve the question with automatic cam chain tensioner so I just converted Diversion tensioner into manual. I have no photos of tensioner but I used original housing, just drilled it, cut 12×1,25 thread and made custom tension bolt.

I shot photos of all setup, when maintened “Eight Ball”.

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