Topic: Can I use F2 pistons and cam in a K6 top end for more performance?

[Terry in Australia]

Well if you said "yes", you're sadly mistaken, so go to the back of the class and write "I think I know everything about CB750's, but I really don't" 100 times.

How do I know this? Well a year or two ago, I installed a K6 top end on my F2 engine, and as the F2 pistons looked to be in good condition, I thought, "Why not?". I mentioned this in another thread, and I wasn't the only one here who had some doubts as to whether or not this would work, and some discussion ensued.

Some folks suggested that I should "clay" the pistons to measure clearances, but I'm loathe to reassemble an engine just to pull it down again, I'm an impatient fcuker at the best of times, so I naturally defaulted to my usual "suck it and see" method that I use for every facet of my life, which goes some way to explain why I'm carrying more injuries than just about anyone else I know, except perhaps, Evel Kneivel.............

Well as expected, once I reinstalled the engine and fired it up, it had a particularly loud top end rattle, that had nothing to do with a sloppy cam chain or overly large valve clearances. So, as has become the norm with my poor old F2, I parked it for another couple of years. Fast forward to now, and I've decided to "for once and for all", get this sucker going properly, so I can sell it to make some more room in my garage, hopefully for some updated workshop machinery, or possibly for a blower for my Triumph Rocket III.

Anyway, I pulled the engine out today, and once I'd whipped the head off, there was no doubt that combining an F2 cam and F2 pistons in any engine other than an F2's, is a bad idea. Not bad enough that I've got bent valves are destroyed pistons, but there's no doubt about the "witness marks" on the pistons. You could certainly "fly cut" the pistons (only on the exhaust side, there were no marks in the intake side) if you wanted to continue to use them, but I'm just going to install some K6 pistons (with the F2 cam, probably) as I've already promised Adam ("Ace" here) the F2 pistons for his engine. Here's a pic. Cheers, Terry.

[Soos]

I've already promised Adam ("Ace" here) the F2 pistons for his engine.

Damn, I got to watch you more often!!
F2 pistons..... Great cb650 overbores......



Too bad, got any more sets laying around you want to part with??



l8r

[Big Jay]

Probably only the valve pocket issues, because when Wiseco made F2 836 kits, we would put them in K motors for high compression, and everything worked.

Did you check cam timing when you did it? If the cam was retarded, the pistons could catch the ex valves.

[Old Scrambler]

Terry........thanks for the info.........now I have to get a few frozen pistons loose.........I've got 3 of those black motors laying in the back of my shed.

[bucky katt]

Terry........thanks for the info.........now I have to get a few frozen pistons loose.........I've got 3 of those black motors laying in the back of my shed.

dont suppose you'd consider selling me one of those F2 cams would you?

[Terry in Australia]

Probably only the valve pocket issues, because when Wiseco made F2 836 kits, we would put them in K motors for high compression, and everything worked.

Did you check cam timing when you did it? If the cam was retarded, the pistons could catch the ex valves.

G'Day Jay, thanks for that mate, yep, the cam timing was spot on. I don't remember seeing the Wiseco F2 big bore kits, but the "Action Fours" F2 big bore kit (one of which I've got sitting in a box in my garage) has much deeper, wider valve pockets than the standard F2 pistons, so if the Wiseco's were similar, that would explain why they would work fine in a "K" series engine. Cheers, Terry.  

[754]

Why dont you just check valve clearance with a screwdriver & indicator... like everybody else does.. you only need the vavlecover off to do it.. on a 750

 clay though will tell you if valve cutout is in right place.

[Old Scrambler]

Bucky........take a look at the Wanted and For Sale pages...........I only have one cam available and having second thoughts.......

Terry and others.........is it possible that the extra duration of the F2 cam is causing the exhaust valves to hit?  It would also be good to know how close the intakes are.  Maybe Frank's putty would tell us that......but I don't think you would really know until the motor is under load and at operating temperature.

I picked up a project F2 with an 836 kit.......it has low compression.  I believe it has the wrong pistons meant for a K motor.  I thought of swithing the heads......or ??

[simon#42]

i have a 836 kit meant for a k motor in my f2 , had to skim 0.7mm from the head to get the compression ratio right , works fine

[eurban]

i have a 836 kit meant for a k motor in my f2 , had to skim 0.7mm from the head to get the compression ratio right , works fine

Any idea as to what compression ratio you ended up with after the head skimming?

[Old Scrambler]

If you got it back to 9 or better.......GREAT!  Thats 1.4mm of camchain to allow for.

[simon#42]

it ended up at just under 10 to 1 [ was aiming for the standard wiseco ratio ] , although the head has 1mm oversize inlet valves in which might make its volume slightly different to a standard head . had no problem with the cam chain length , the adjuster still had plenty of adjustment left , did use an adjustable cam sprocket but to be honest i doubt that is strictly necessary

[MRieck]

i have a 836 kit meant for a k motor in my f2 , had to skim 0.7mm from the head to get the compression ratio right , works fine

Info like this is very helpful. Solid numbers with...later in the thread results. No "what if??" rhetoric.

[paulages]

Why dont you just check valve clearance with a screwdriver & indicator... like everybody else does.. you only need the vavlecover off to do it.. on a 750

 clay though will tell you if valve cutout is in right place.

the problem i've found with this frank, is that you have to rotate the engine through and check in several different places, unless you know for sure where the point of closest contact is going to be.... TDC won't really tell much, as the worry points will be when the piston is chasing the exhaust valve closed, or when the intake valve is chasing the piston when opening. depending on the grind and timing, you never know exactly where the closest point of contact is gonna be. i measured the valve clearance on the 718cc engine in 5 deg increments (around the beginning of the valve lift) when i reassembled after i had valve contact from timing issues, but i'd still always clay up a motor when assembling for the first time.

only gotta do one piston... really not that big a deal.

[754]

I dont think its real hard to determine where it has to be to check it.. where I got it from was the  guys working for RC... They got tired of claying them.. so put em in the right place, lift rocker & check for min clearance..

 it was probably Slim Moffat, or Vance or Hines...

[MRieck]

I check 10 degrees before and after TDC

[bwaller]

When the wheel & indicator are installed it only takes a couple minutes to check. Seems to me the most positive accurate approach. Clay can be difficult to get a precise read.

[Ace]

Finally I get to see what's left of my pistons.  At least now Terry they will be a bit lighter in the mail. 

[Terry in Australia]

Finally I get to see what's left of my pistons.  At least now Terry they will be a bit lighter in the mail. 

Ha ha, fear not mate, they're fine. Cheers, Terry.

[dave500]

valve overlap is the closest pistons come to the valves.,another word we use here in australia for clay is plasticine.

[PeWe]

I have a K6 with modified engine. One of the modifications is F2 inlet valves (2mm bigger), cylinder head is ported and adapted to 836. Works very well!

But the pistons for a F2 have a different dome. There is a good description at Dynoman. http://www.dynoman.net/bikepages/F2/pistons.html
As the 1977-78 Honda CB750 F2 has an 18% larger combustion chamber to compensate for the larger valves, through the years many people have installed the standard 836 (ex: Wiseco K836) kits for the 69-76 engines only to discover that the compression was too low to give them the extra power that they were after.

K6 standard cylinder head might have too small combustion chambers for the F2 pistons with higher domes.
(Or a higher compression with good result)

There are many different cam's available for not much money to increase the performance.


Now I'm curious too about the pistons versus F2 inlet valves.
Time for surgery and my 3:rd total engine operation taking everything apart. Last time 1985...
I'll check the details when open the engine after soon 20 years of no use. The bigger valves might have an effect on the clearance when choosing cam which I plan to replace with a hotter one . (I have Action Fours SS-1 cam)

I had Russ Collins forged 836 pistons that were adapted to the engine by the tuning company here, the piston heights, cuttings for valves were not machined from RC.
2 of them got a meltdown due to my carbs, Mikuni Smoothbore 29mm that got too much air due to damaged inlet Rubbers. (wrong rubber mix).

Replaced the carbs with original (main jets 145), pistons with Action Fours 836 which might need new rings if I keep them, the compression might be too low. I remember that the RC pistons gave much more compression and torque, when the Action Fours worked well for touring.

I have seen here that CB350 65mm rings will fit. I hope it will when the rings must be replaced by new when opening the engine.
My interest have woke up again thanks to this forum, cycleexchange.net and Carpy's cb750cafe.com