72' engine component question

[PONYJR]

Hello all. I recently have acquired a 1972 XS2 motor. I have verified this via the serial number, parts diagrams, and all other reference materials I have found.

From what I can tell, this is a hodge-podge of parts between 256, 306 and 447 engine parts from Yamaha. My engine cases have a 306 number, rods a 447, 20mm wrist pins (yet again 447), 256 cam, and a mix of 306/256 crank parts.

I have a set of big bore 256 pistons I plan on using, so I have to swap out the rods to get the 22mm wrist pin. I know this can be done. What I need to know is what else might I have to change? Do I need to stick with the 256 cam if I'm swapping out the rods?

 

[5twins]

The cam and rods aren't related. It doesn't matter what cam and rod combo is used. What does matter as far as cams and cranks go is matching the sprockets on those components. The 256 motors used different sized sprockets, cam chain length, and chain pitch compared to the 447 engines. You may also need to consider your cam chain tensioner assembly. If it hasn't been swapped out and is still the '72 version, it will have a sprocket on it made to fit the 256 cam chain pitch.

 

[gggGary]

IMHO I would not use the big pin pistons. Bite the bullet and buy 20mm pistons. I think you are lucky if you have the later rods in good shape. Think on it a bit, what is the likely reason "late" rods are in an early engine? This is coming from a guy that has a brand new 22mm 750 kit with Wiseco pistons and match bored cylinders ready to install and a 1100 miles 72 motor to install it on!
The deal is; Yamaha first built the XS650 motor with 22mm roller bearing rod ends due to their 2 stroke engineering background. They quickly found out roller bearing small ends are not such a good idea in big 4 stroke motors. The quick fix was to change to plain bearing 22mm wrist pins. But that 22mm hole left little room for enough metal in the rod and the 22mm rods tend to stretch under even moderately hard use, causing wrist pin slap, this is not good, in severe cases the pistons will hit the valves!
That's why they changed to 20mm pins with more meat on the rods when the 447 motor debuted in 74. By 74 the 650 motor was a build and sell cheap bikes unit, not cutting edge. XS650 re-engineering was; use existing tooling, slap a band aid on known problems and crank out low cost motorcycles. It must have bugged them to no end how popular the XS650 stayed even though much "better" machines sat alongside them in the showrooms.

 

[650Skull]

Here is a nice story about an XS1 from the Aussie 650 club. The pics tell the story of a problem that ggGary talks about.

This was not unusual but then young people and motorbikes = fast = these sorts of problems

 

[PONYJR]

I appreciate the advise, but I will continue to use my 256 pistons. These are very rare nearly perfect Shell racing pistons that I have been given. I'm already in contact with Hugh about building my crank.

While we're at it, any recommendations on a decent place to do the sleeve and head work? Suggestion on a cam?

 

[TwoManyXS1Bs]

That 'blown out' engine case reveals something I've never seen before. Looks like the front oil gallery is fitted with a tube of some sort.

Do you have rings for those '256' pistons?

Hoos Racing may be able to do your machine work:

http://www.hoosracing.com/

 

[PONYJR]

I have the original rings, look to be in "ok" shape, though I am actively looking for a new set. If I cannot find a source for them, I have a few places that have said they could manufacture new ones for me.

I did find the Hoos guys. Prices seem reasonable enough. Still waiting to hear back from a few shops I have emailed.

I have contacted CP about a set of their rods, as they do not list them in their catalog, and I could not find any online.

Thanks for the help.

 

[gggGary]

Looks like the front oil gallery is fitted with a tube of some sort.
http://www.hoosracing.com/

Saw that also. Guessed it was easier to place a tube in the mold than try to bore out the gallery later.

 

[PONYJR]

It appears that my '72 has the wrong kind of cam chain adjuster considering it has a 447 crank and cam installed. I have found all kinds of documents and information on the different types of adjusters, not not really anything about which type would "retro" fit to others. Any ideas on which type would work best for me?