CB500T top end, lots of questions

[Helve]

First time rebuilding the top end on a bike, so I'm not really sure the best way to go about diagnosing my issue (low compression, about 150psi on each side, white smoke from left cyl exhaust). Pulled the heads and found this. ??? ???

Pulled the cylinders and found some light wear marks but no scoring. Cylinder base gasket was covered in oil, assuming it may have been part of the problem.

Pistons seem OK, but I'm not sure if the deposits above the rings are normal. Indication of anything bad? Also, I ordered new rings but I'm not entirely sure if it was a necessary purchase.

Any suggestions as to moves forward from this point? Ordered new gaskets as well, so I'm planning on taking the cylinders to a machine shop to have them inspected but I'm looking for moves to make in the mean time. If you need/want any more details or photos to help me diagnose the issues, let me know.

 

[xb33bsa]

mark the valves on one cylinder just a little mark right in the center with a dremel works great it is only a 2 cylinder bike so you only need mark one side
i always mark the left side (sitting on bike) because that is no. 1
then the very first thing is to check the valves and guides if they are worn out there is no sense going further until you remedy that
all you need is a dial indicator and a micrometer and the factory service manual has the procedure
this will keep you busy for a while and taking the pistons and cylinder to a good machinist is a great idear as well again mark the left,no. 1 piston

 

[Helve]

Gonna pull the heads apart today. Got a local shop that can machine the head to install new Schumann bronze valve guides and seals, and bead blast the ports/chambers. Gonna have to get a quote but I assume it's within my budget. As far as the cylinders go, anything I should be concerned about based on the photos I provided?

 

[xb33bsa]

they look like they need boring over next size, new pistons

 

[xb33bsa]

bead blasting the head is ok if you carefully plug any and all threaded holes and oil passages
the beads can be near impossible to clean out of a passage

 

[ProSimex]

take one of the piston rings off and stick it in the barrel at different points. Measure the ring gap with a feeler gauge, ill bet they are wider in the middle then top or bottom. I would bore em out and fit new oversized pistons, just looking at the scoring on the pistons and walls.

 

[Helve]

Got the cylinders honed this afternoon. Worst of the scoring is shown in 2nd pic.

Rings show .012" clearance top, middle, and bottom.

Thoughts?

 

[Flugtechnik]

My opinion, always put in new rings. If a good honing cleans everything up and you still have clearance on your new rings, then no need for an overbore.

Check all of the cam chain guide wheels. I had one with a busted pin. Replace all of the seals and gaskets you can.

Be careful if you get the valve seats recut. You will likely need to shorten your valves slightly.

Carbon buildup is common. Don't worry about it. Clean it and move on.

Congrats on the bike.


END OF LINE

 

[Helve]

I done did goofed. Had my torsion bars all spread out on my workbench, in a way that I knew where they had come out of and since it's a shared space someone moved them all into a pile to make room to work on something else. Any tips on how to test these things to make sure they go back in the right place?


On another note, took my head into the machine shop today. Machinist said he'd be able to look at it monday. Not 100% sure if I'm going to have him cut new valve seats and match the valves but I'll wait for him to let me know what he thinks. The rest of my parts for the rebuild should be here wednesdayish, so this weekend is prep work time. Cleaning off old gasket material, prepping gasket surfaces, etc. Super excited/nervous to rebuild this thing. Fingers crossed it all goes smoothly...

 

[xb33bsa]

it doesn't matter don't worry about it
take the factory service manual to your machinist as well so he can see all of the specs

 

[frogman]

Torsion bars should have an A and B stamped on the side of them. Each cam should have an A and a B. The A's and B's are also put in on opposite sides.

Picture for reference, some of the springs may have arrows on them depending on the age of the engine. Arrows should go UP when replacing.

 

[Helve]

Thanks man. Really appreciate that. Doesn't matter which side each A or B torsion bar came from I assume?

 

[frogman]

Ideally they should go back where they were but it no it shouldn't make much difference.

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