Topic: 750F piston and engine ID

[capnspif]

Hi all,

I have a few questions on motor parts and identification I hope someone can answer.

I recently tore down a motor from a '78 750F I'm rebuilding to discover the cases were cracked around the primary drive. This motor has a head that's stamped 410 with flat top pistons stamped 300, they have valve relief in the piston tops. Bore is stock, right around 2.402.

I picked up another parts bike, same '78 750F, and just tore down that motor to get the upper/lower cases. Motor2 also has a 410 stamped head but has domed pistons stamped 410, with very small valve indents, same 2.402 bore.

Cylinders, heads and valve sizes seem to be identical between the two motors. Motor2 seems to have much beefier valve springs but looks like it has been torn down and reassembled at some point where motor1 looked 'original'.

The question is, what do I actually have here? If Honda didn't change pistons in the same production run, are the domed pistons stock for the '78 750F3 or should I use the flat tops? My intention was to rebuild everything from motor1 with the cases from motor2. Now that I have it apart I'm considering one of the big bore kits. Are those pistons from a typical Wiseco kit for example, compatible with the F motor/head? Basically, are the K pistons and F interchangeable? From what I've read the F valves are hard to come by, so should I consider a K head or are there other options?

Lastly, any recommendations on paint/powder coating the cyclinders/head/case? The original black coating on both these motors is worn/aged and I'd like to redo. Any recommedations?

Any advice or input greatly appreciated.

Thanks,

Mike
mike@capnspif.com

[Lumbee]

...hey Mike this is a good post.  I am very interested in the responses.  I have a 78 750K and  78 750F.  I just rebuilt the top end on the K and am planning on rebuilding the F in the new couple months (I work slow  ).  I can tell you the K pistons are flat also, there is some interchangablilty in the F and K models.  For instance I had a bad vavle guide on the K head, so I rebuilt it with an F head/valve train.  I was told this will work OK.  I have yet to confirm it as I just finished the rebuild and do have the motor running yet.

[MRieck]

 The 77 and 78 F's have domed pistons with shallow reliefs. This was done secondary to the larger chamber necessary to accomodate the larger valves (basically the 3mm larger exhaust valve as a 34mm intake valve will fit on a stock K head). These bikes also had much beefier valve springs. Someone was in the engine and was mixing parts around. The late F head on a K will only work if you use the F pistons or something with a comparable dome. Personally I think it is better to swap out the late F parts for a K head and early (75 or 76)F pistons orbetter yet a Wiseco 836 kit. The reatiner/keeper problems was well as the lack of aftermarket springs /retainers make the late F stuff undesireable IMO. There are plenty of aftermarket valves ,springs, Ti retainers, etc available for the K.

[Lumbee]

Mrieck...you said an F head will NOT work on a K without the F pistons?  I was told this would work.  Why do you say this?  Should not be clearance issues since the K pistons are not domed, and the F chamber is larger.  Loss of power/compression maybe?

[capnspif]

Thanks for the insight. As far as rebuilding one good motor, I guess I'm looking at using the bottom end, con rods and cylinders from the '78 F, big bore pistons or the stock flat top K pistons, and finding a complete K head to top it all off? Assuming the "410" heads I have now are the F heads - I guess I could measure a valve to be sure. Will a K head match up OK with the F cylinders and this set up in general? Or would I need to get the K cylinders also? Anyone know what the 836 big bore kit adds in terms of horsepower with a stock head? Thanks for all the advice.

Mike