Cylinder Heads Compatibility

[Polkahero]

Would a cylinder head assembly from a '79 XS 1100 be compatible with an '81 model?

[rover]

heads

The later model has larger dia valves, and a different compression. It has been done. Do a search.

[trbig]

I always hated the "Do a search" answer to my questions. Our search tends to lead you to a thousand links that somewhere in them may have the word you're looking for and no way to know which page it could possible be on with each thread listed.

Yes, you can use an early head on a late motor, BUT.. the consensus seems to be that you need to use the cams that pertain to the head and that it's best to use the timing mechanism for the head/cams you plan on using. You should get a little more HP and compression from doing as you were asking, but it's easier to just put back in what you had. Plug and play.

[Polkahero]

I always hated the "Do a search" answer to my questions. Our search tends to lead you to a thousand links that somewhere in them may have the word you're looking for and no way to know which page it could possible be on with each thread listed.

Yes, you can use an early head on a late motor, BUT.. the consensus seems to be that you need to use the cams that pertain to the head and that it's best to use the timing mechanism for the head/cams you plan on using. You should get a little more HP and compression from doing as you were asking, but it's easier to just put back in what you had. Plug and play.

Reason I was asking is that I'm selling a cylinder head from a '79 model and someone with an '81 model was looking to purchase it. I told him I wouldn't recommend doing so as the cams and valves are different but you're saying it could be done without any adverse results?

[ViperRon]

The 79 head you have will not use the cams or valves from 81. Mechanically the older 79 head and parts in it are completely different from an 81. If you put the complete 79 head on containing same year valves and cams it will mechanically work with no problem. To get proper performance the timing vacuum advance should be changed to the same year as the 79 head. This is what TRBIG said do you understand?

[Polkahero]

The 79 head you have will not use the cams or valves from 81. Mechanically the older 79 head and parts in it are completely different from an 81. If you put the complete 79 head on containing same year valves and cams it will mechanically work with no problem. To get proper performance the timing vacuum advance should be changed to the same year as the 79 head. This is what TRBIG said do you understand?

Got it, thanks. Is the timing vacuum advance part of the pickup coil/governor assembly?

[ViperRon]

There is a mechanical advance springs and weights and also a vacuum advance mechanism under the left side cover sitting on bike those give different trimming curve to work with the valves and cams.

[MaximPhil]

There is a mechanical advance springs and weights and also a vacuum advance mechanism under the left side cover sitting on bike those give different trimming curve to work with the valves and cams.

Hi Ron,
If he changes to the mechanical advance does he need to change the TCI box as well ? There is electrickery in there so beyond my knowledge or pay scale.
Phil

[ViperRon]

To make it work you would not need to change. To make it work well the trimming box TCI would need to be changed from a 4R0, the wiring would need to be changed as well as there are issues with what type coils to use and wiring of the ballast resister.

There has been discussion of the easiest way to get the best performance from the 78/79 head on an 80 and 81 engine and opinions vary. Some say to vary the trimming start point will closer match the curve others disagree. So to put the 79 head on and make it work as well as the 81 I imagine just replacing the head completely and centrifugal advance would get you there. To try and gain max hp would involve turning it into a 79 completely in the electrical department.

The 79/79 bikes were mechanically engineered for a higher output part of which came from the cams and trimming advance in a different curve.


So if you want all specific information I believe one person here on the forum to ask is 3Phase as I know he had the specifics of all the different advance mechanisms and wiring for the change. Now 3Phase may not be the only expert on these functions but I have seen his posts here. and you can do the searches or just send him a note.

So the question is whether he's looking to make it work or modify it for power. You going to sell him the head that works or reengineer bike for him???????

[tcoop]

I currently have a 79 head on an 80 LG. I am using the cams that came with the head. The advance and TCI are the ones from the bike (80LG) It runs very well with no issues to date.
The 81's advance may be different from the 80's but I do not know for sure.

[skids]

You just can't argue with success!

[TopCatGr58]

Hey Tcoop,

The 80 engine uses the newer designed pistons, but still has the same older mechanical cent. and vac/. advance and similar TCI, so that may be part of why it is able to work.

The newer 80-81 pistons have larger/wider valve recesses, and are not as domed, and that is part of why the later engines have a slightly lower compression ratio of 9.0 to 1 vs. the 78's 9.2 to 1.

As was stated, sounds like the trick is using the same year cams as the head, the electrical ignition timing may not be as critical??

T.C.