more 79 to 81 compatability questions

[yamahaluke]

I need to know if I can mount 79/earlier pistons and jugs to my 81/newer crankcase and con rods? any thoughts would be helpful

[crazy steve]

Yep, you can....

[DGXSER]

I know the 80-81 models had differences in the pistons to make up for the differences in the heads with the bigger valves. I think the dome was a little taller to change the compression. So I am not so certain this swap would not change your compression.

I know Dan Hodges has sorted through ever option of interchanging the pistons, heads, and cams. So read through a few of his post to see what he found. He forgot more about these engines than most any of us ever knew I think (meaning he snows his sh!t).

[crazy steve]

I probably should have expanded on my answer a bit...

Your only concern will be valve clearance if you're using the '80-81 head due to the larger valves. If the valve reliefs in the tops of the pistons are the same (and I believe they are), no issues. If the '78-79 pistons have smaller reliefs, then you need to check this for a minimum of .020" clearance. This would be a rather lengthy involved process, so if in doubt, I wouldn't use them. If you use the early pistons/late head, you'll also take a small hit on compression ratio but you could probably make that up power-wise by putting in a bit more initial timing. Most would never notice the difference.