DOHC Big Bore -- What do I need?

[jwaltmire]

Hey friends,

Some background: I have a 79' 750K which has a hole in the cylinder block (sawed through by the cam chain). Otherwise, the bike is in good shape and runs surprisingly well aside from the oil leak and cam chain rattle.

I've ordered a new block on eBay, and am considering boring it out and using a Wiesco kit to get more power/better compression while I have the top end apart anyway.

Can anyone give me a definitive list of parts that I would need to swap/upgrade to make a big bore kit work on my K? I already plan on replacing the gaskets, pistons and rings (obviously), and cam chain. Do I need a different or modded cylinder head? Different cams?

I've already done some searching through the forums and haven't been able to find out what has actually worked. Any help would be great!!

[TOOLS1]

These are the piston kits I use when I overbore engines. Excellent quality, and a lot cheaper then Wisco.

http://www.ebay.com/itm/79-82-HONDA-CB750F-823cc-BIGBORE-PISTONS-KIT-65mm-PISTON-10-CB750FBB-/162355320744?vxp=mtr&hash=item25cd2197a8

As for the engine needing anything else, nothing else is necessary, but a CB 900 cam is a direct swap, and does improve performance.
TOOLS
BTW. I bore and hone cylinders, and do valve jobs.

[jwaltmire]

Thanks, Tools! This is awesome news. I've ordered those pistons-- $150 is much more manageable than $500 with the money that's already going into boring, new gaskets, cam chains, and anything else I'm liable to find in there once I open the engine up.

I've also managed to get my hands on some CB900 camshafts. Any best practices when I drop them in? This is going to be my first foray into engine rebuilding-- using my Clymer manual as a bible.

We've got several machine shops here in Portland that should be able to do the engine boring for me, but I appreciate the offer. I have no idea what state my valves are going to be in.

[TOOLS1]

Portland, my friend Becca just moved there. I would recommend you down load the factory service manual from our manuals thread here. It is a lot better then the Clymer/Haynes manuals.
TOOLS

[Truck]

DAAAMN....wonder how much it costs to send an engine to KC???

[deskinthewoods]

"I have no idea what state my valves are going to be in." I would imagine they would be in Oregon...Sorry, couldn't resist...

[jwaltmire]

Hah! You got me there.

Thanks for the heads up about the factory manuals. I'll definitely use them. If there's any interest, I'll keep posting through the rebuild process and share the final result.

[jwaltmire]

Hey Tools,

When boring the cylinders, what clearance do you give these pistons? I purchased the ones you recommended and can't find any good info on them.

Thanks!

[jwaltmire]

To clarify: Some sources say as low as 1 thou for the no-name pistons, and others (like Wiseco) give the pistons 2 thou.

Might just try my luck in between at .0015" and assume a light break in period?

Anyone had a good experience with a certain clearance?

[TOOLS1]

0.002"!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
TOOLS

[jwaltmire]

Done. Thank you!!

Another question-- when running this big bore setup (65mm pistons and CB900 cams), is there any need to go with bigger jets in the carbs? I've seen some kits being sold on aftermarket part websites. Do bigger chambers need more fuel to combust?

This thing is looking like it should come together this weekend.

[TOOLS1]

Not necessarily. Big chambers can produce more vacuum, and therefore pull more fuel through the jets. You will have to put it together and run it to find out if it will need a jetting change. I kind of doubt it will.
TOOLS