650 cyl head onto 750 engine?

Hello,
I bought a unusable 81-83 Seca 750 cyl. head from ebay , but, ever-perservering, I see a couple heads for a 650 non turbo Seca, same years... Are they interchangeable? I think I read somewhere on XJBikes they were identical for both cc/size engines?

 

I'm thinking not; the service supplements I'm reading show the 650 having a bore of 63mm while the 750 has a 65mm bore.

 

There are some detailed 750 specs here that may be helpful: http://www.motorcyclespecs.co.za/model/ ... a%2081.htm

FWIW, the valve sizes are the same, but the cams are different. On the other hand, the stock compression ratios are the same, and the bores are definitely different, so it seems unlikely that the heads are the same. If the crown of the piston doesn't intrude into the combustion chamber, you might be able to install the 650 heads and boost your compression, but....?

 

Chacal says they're the same except for the cams, so I went back to the XJCD and looked it up again. Unless I'm looking up the wrong models (650RJ vs 750RH <> 750RK), the supplements say different.

 

Motogrid's IPB lists two very different SKU's for the heads:

750 = 5G2-11101-00-00
650 = 4K0-11110-00-00

 

The 4K0 is for the non-YICS heads, which ARE different. The 5G2 is for the YICS heads, both 650 and 750, although you'll find some other numbers for some of the 750 Seca heads, they are just due to paint color variations.

 

I was comparing an '82 XJ750RJ (my bike) to an '82 XJ650RJ.

I didn't realize the '82 650's were non-YICS. Sounds like one extra thing skizex needs to check for.

Chacal, how do they keep the compression ratio the same on the two bikes? Is the piston more crowned on the 650, or does the 750 stroke not go "all the way to the top"?

Thanks,
Paul

 

By the way, what's "unusable" about the one you got, and what's wrong with the original you're replacing?

 

Paul, the '82 650RJ Seca was a non-YICS motor. All other '82-later 650 heads were YICS heads, just like the 750's.

That's a good question, though. One of these days I'll get out all of the little bits and pieces (cranks, rods, pistons) between a 650 and a 750 and compare, to try and figure out how it was done; the 750's also have a longer stroke, but maybe not enough to make up for the bigger volume? The pistons, at first glance, seem to have the same amount of "dome", but of course the pin placement within the piston may also be off some. Really hard to tell without taking uber-accurate measurements.