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I may have posed this question previously:( just checked and yes, I have, but with no definitive answers)

what do I need to know and/or what is a good resource to figure stock cylinder & piston compatability from other years with a '77 /7?

For example, donor pistons & iron cylinders from, say, a '78 or '79, while retaining existing heads.

Foreseen issues- wrist pin circlips vs spring clips
cylinder base size differences
cylinder base o-ring vs none

The main compatibility issues I would be concerned about would be the proper mating of the piston dome shape with the contour of your cylinder heads and even that is not likely to be a problem - especially if you end up with the (most common) flat topped 8.2-1 pistons. Unless you're going racing or are going to be boosting the CR via milling/turning I'd not worry about it.

The pin-to-rod connection will be the same, regardless of what pistons you end up with. The inside circlips are a PITA and if you really hate them (you have them now if your bike is stock), you can have the pistons carefully milled to accept the outside clips on the later style pins - I've done that, it works fine.

The base of the barrels with the bottom large o-ring have a very small step that will interfere with their fit to your crankcase, at least this is true for the Nikasil barrels. You can either have it turned off or (warning PITA!) scrape the cases carefully to eliminate the interference.

O-ring or not I'd just carefully clean the base of the barrels and the mating surfaces on the case and apply a very thin, even coat of Permatex orange bond (it smells like RTV - I'm not where I can go look at the exact labeling).

Thank you John.

Yes, I did recall the bases w/ o-rings are somehow sized differently. All the reading I've done leads me to believe the 1977 was just after & just before a few minor but important changes which is why I'm unclear about direct swap donor barrels. It may be that '77 only fits a '77?

I don't think that's the case ('77 barrels being unique) but if you find out differently, please do post that info for others.

Delving ever deeper, turns out '77 was a "special" year -at least for pistons! Apparently the configuration, high compression, was used one year only. So, for anyone else looking,it seems that the heads, & pistons of this year are not easily interchangeable with others.

I assume,therefore, the easiest thing is to swap the whole top end.

Well, 1977 was special for some bikes but not all of them.

The 77 RS and "S" bikes were available with high compression big valve heads and squish band pistons, some with 40mm exhausts and some with 38's.

You'll want to either check around to see how your plain jane bike was configured or simply pull a head and take a look but AFAIK, relatively few of the '77' bikes had the 70hp engines and the ones that did were the "S" and RS models.

I'm currently running BV heads with new Niks and new high compression non-squishband pistons but I'm certainly leaving some HP on the table that I could pick up if I could find the correct Squish Band pistons to fit my cylinders. I'm keeping my eyes open but I'm not counting on it. Those parts are as rare as hens teeth.

Take an hour to pull a head and let us know what you have in there!