Topic: 750 F2 into K4 valve train swap.

[Don R]

 I did search first, sorry if it's covered somewhere,
I'm ready to mill my 750 K4's ported head and then have the new stock valves installed for the Ebay 836 kit. Then I unexpectedly got most of an F2 black engine cheap, now I'm thinking I can use the cam, retainers and springs from the F head. Also, maybe the larger intake valves.

Question is what should I know before attempting to use the F2 parts?

[Retro Rocket]

PM Mike Rieck....

[Terry in Australia]

I wouldn't do it Don, the parts you want to use were the downfall of the F2 and F3. The cam design was poor, causing the valves to snap shut really hard, and this was exacerbated by the overly stiff springs, and on top of that, the valve stems were too soft, causing them to "mushroom", and the guides would get loose in the heads. All of the above contributed to accellerated top end wear, and the occasional dropped valve.

Rex Wolfenden, Australia's top CB750 based race bike builder once told me that the best head setup was a K2 head with a larger inlet valve but stock size exhaust valve. He said that the F2 inlet valve size was fine, but the exhaust valve was too big, and didn't flow as well as a K head. What should you know before using F2 parts? There are more F2's with dead engines out there, than there are K's...........

[johno]

Hi Don,
The way your going you'll end up with the chevy head on mate, that would give some good flow rates.

Don you can fit the inlet 34 mm F valves into a K head,  but mate they are on the limit with the valve seat insert. the valve sits right on the edge of the insert and you are in stranger danger territory for wear or failure.

You will need to sink the valve into the head deeper to get valve to valve clearance, then throat out and blend into the port.

The valve collets are different , re adjust by shim the spring heights to maintain the installed height pressure.

I would agree with the boys and leave the exhaust the same size as std K

The bigger valve will give you extra flow at low lifts but if your just tidying up the port the advantage will be minimal.  You then will have to live with the extra spring pressure to control the heavier valve and the rapid wear on the skinny insert seat.

I think the main point is which head does your new pistons suit K or F  , will effect the comp ratio dramatically and piston valve clearance etc

I would be guided by the pistons you have all ready purchased. build top end to suit them.
ciao Johno

[MRieck]

As noted...the F2 valves are shorter than the K valves...you would have to really sink them into the head to get the right stem height. Than it would flow like crap and increase the chamber size a good bit. The retainer/keeper system is different as noted and not as reliable. Use a 33.5 intake valve, have the seats bored to .9 of the valve OD and backcut the the intakes....it will flow better than the stock 34mm valve in the stock F2 seat (given identical port volume).

[Don R]

Thanks guys that made it easy for me to decide to leave it alone. I dropped the head off to get milled .010. I'll expend my efforts on guides and seals. It's getting close, now to address the cam timing.

[johno]

Don,
 I would love to know about your ebay pistons specs,  any chance of posting a pic ?   I hope your not a technosaurus like me 
ciao Johno

[Don R]

Don,
 I would love to know about your ebay pistons specs,  any chance of posting a pic ?   I hope your not a technosaurus like me 
ciao Johno

Here is everything I know about them,
http://forums.sohc4.net/index.php?topic=89779.225

[johno]

Thanks Don,
After the sparky thread I guess the whole world knows the pros and cons of those babies now. You have put my mind at ease Don I was worrying about hi top F 2 or 3 pistons in a K head.

For what your doing Don they sound fine, fresh overbore , valve freshen up, tidy up the ports, it will be an enjoyable  bike to ride Don.  Snappy throttle , smooth as, sounds like some great riding coming up. 

Cheers Johno