2009 Monster 696 Engine Rebuild

[rohan696]

Hello everyone,

I'm new here, but have referred to some posts from the forum in the past as a guest.

My M696 recently suffered engine damage while I was out riding- to diagnose, I started by opening the timing belt covers and both tensioners were broken and the belts frayed a little, but not broken. After that, I pulled the horizontal head off and surprisingly everything looks good in there! The vertical head, however, is in bad shape- one of the valves is broken, the piston has a hole in it and the spark plug damaged.

The engine is now on the bench and I plan on rebuilding the vertical head. I know that I will have to replace the piston, cylinder, valves, probably rocker arms, head, guides etc., and will also check the connecting rod and probably the crankshaft as well for damage, but is there anything else you guys suggest that I look for or keep in mind?

Any help is welcome and greatly appreciated! Thank you, everyone.

P.S. I'm aware that I was an idiot and it was my bad for not checking, adjusting and replacing the timing belts on time, so spare the bashing, please, and just try to help if you can!

[Howie]

Off hand, what I can think of now is remove the vertical cylinder and make sure it is not damaged before spending any money on parts.  I doubt the crank or rods are damaged.  Any idea of engine speed when the failure happened?

[rohan696]

Thanks. Yes, that is exactly the plan. I'll remove the vertical cylinder and see what's going on. The piston is at TDC so if I'm lucky it would've stayed there when the bike broke down and the damage wouldn't have gone past it into the cylinder walls..? Luckily I was cornering stuck behind some cars in the canyons when it happened, so RPM wouldn't have been over 3,000, if that.

By the way, the rocker arms look fine through the valve covers, but the intake valve broke off, so I'm thinking of removing and inspecting them carefully for cracks, and maybe the cam as well. What do you think?

Exhaust valve isn't broken, but has some scratches because of what went down in there. It also seemed odd to me how the spark plug was damaged, but my guess is the broken intake valve is what hit it because I found the valve flipped upside down and stuck on the piston.