Late last year had my back cylinder head gasket go out. Pulled the heads jugs and pistons to rebuild the top end and noticed that the back piston was cracked and that head had marks from the piston slamming into it. My question is where to I start to troubleshoot to find out why the piston started hitting the head. Maybe crank bearings are shot? The engine is a 2002 efi TC88B with high compression 95 pistons. Bike is a salvage bike so this is my first time digging into it. Thanks

 

Just recently posted on our Facebook machining needed on early S/E cast pistons in a 95", if heads are surfaced, w/pics.
Scott

 

I can't say for sure if the heads have been surfaced or not, but I'm not thinking it's a problem with initial clearance between the head and piston. If it were I would imagine that in the 3 years I've had the bike I would of had problems sooner. Also would think both pistons would show the same marks? It's only on the back cylinder that had the bad head gasket.

 

The rear cylinder gets hotter, so it will see more thermal expansion than the front which will explain what you're seeing.

 

So probably nothing wrong with the crank? Bike has about 25k on it would it be a good idea to take a look at the bottom end just to do it while the bike is tore down

 

Some photos of both piston and head would really help. Kind of hard to visualize the piston hitting the head before contacting a valve. Another thing that isn't computing, for me anyway, is that you pulled the head to replace a leaking head gasket and then discovered the cracked piston and witness marks on the head. I would have thought that if the piston was making contact with the head; you would have known and been pulling the top end down to figure that out rather than finding such a condition accidentally.

 

For peace of mind, I'd pull the cam chest open and put a dial indicator on that pinion shaft.
Scott

 

Crack is in between the rings kinda hard to see

 

Getting into the way back machine I can tell you this from that vintage. The cast pop up pistons will hit the edge of the combustion chamber if used with an SE head or a stock head that was decked - that is why the SE catalogs said to use the forged pistons if using SE heads back then.

Soooo. While the top end is off check for up and down play on the rod and check runout as Scott says. However, on that vintage I highly doubt you have a runout problem. However the first couple years of Betas were known to spin balancer timing gears. That won't cause your problem though.

Check the rod bushings too.

If it all checks out my money says you have the pop ups with shaved stock heads or SE heads (which will have the SE logo on them)