Topic: Adapt "K" head to work on "F" motor??

[BlackMax]

As some know, I've got the usual problem with my "F" ('78) head, that being a valve job that resulted in finding huge tolerances between stem and guide, so much so, my mechanic assumed the wrong valves were in there.

Anyway, Option 1 would be to  completely rebuild the existing head including new guides, valves, retainers and cut seats, by sending it out to a cylinder head shop.   The cost for this should be pretty steep; plus, who knows if something else goes wrong.  When you add up valves, seats, guides, retainers, springs and labor.....surly I'm in for $1200 or more just on the head.  It would be less if I re-use my valves and springs and retainers....but I just don't know if they are good, yet.

Option 2 would be to buy a new F2 head, totally built and ready to go with 100% new parts (everything).  I have a line on something just like this; but, it's expensive, however  maybe less than rebuilding the existing head.

----->  Option 3 would be (and I don't know if this is feasible) to adapt a good "K" head to my F cylinders.  Can this work?      If so, what's the process??

[GammaFlat]

I'm not sure about adapting a "K" head - I'm thinking you're in trouble with chamber volume... 

How about option 4: Find a decent "K" engine.  77 and 78 "K" engines are theoretically the same as a pre-77 "F" engines.  A little paint here and there... hmmm.

[754]

What is a good head worth to you, and where are you?

[Jerry Rxman Griffin aka MuthaF'er]

If you use a K head you'll need K cylinders also. You should be able to find both relatively cheap PROVIDED you're not in a major hurry.

[fuzzybutt]

k pistons and k head are a matched set

[Bido]

While on the subject of F Heads I have a 78 F3 does anyone know if there was a factory recall on this problem or if there was a production fix I have around 25,000 miles and my motor shows no sign of a problem what should I be looking for?

Thanks
Bido

[Geeto67]

While on the subject of F Heads I have a 78 F3 does anyone know if there was a factory recall on this problem or if there was a production fix I have around 25,000 miles and my motor shows no sign of a problem what should I be looking for?

Thanks
Bido

It is not a recall, just a design flaw and you should be looking for oil smoke out your exhaust. Honda increased the valve size in the 77-78F head but did nothing to shore up the valve guide. If the bike has been taken care of they start to wear out at about 30K miles. If the bike was beat up, less miles. The solution is to have new guides fitted by a machine shop. Dynoman sells the guides at dynoman.net

Blackmax - the 78 F head has a larger head volume and larger valves. To compensate for the larger combustion chamber volume the F pistons have dome shape to them to keep compression. If you put K pistons in an F engine you drop the engine's compression ratio significantly, if you put F pistons in a K engine you hit the valves. If you want to put a K head on your bike then I suggest you find K pistons, and I highly recommend (since you will have the engine apart anyway) that you bore it out to 836ccs to get back the power you will lose in switching to a more restrictive K head.

Also you are going to need a 1973+ K head if you decide to do this as the oiling passages in the cylinder and the head changed in 1973.

If all that was wrong with the head was the guides, buy the guides and have a machine shop install them, should only be a couple of hundred and not the $1200 you were quoted. At Dynoman a set of 4 cb750F2-3 valves is $30 (you need a set for intake and a set for exhaust so figure $60). You can probably reuse your springs and retainers (provided none are cracked), but if you wanted new springs it is only $65 for a full set. The guides are $70. Titanium retainers are $200 but as long as yours are not cracked you can reuse the ones you have. None of this is hard work and the only thing intimidating might be lapping the valves (easy).

edit: I should clarify - when I say K head I mean 1969 to 1976. The 77-78K motors are supposed to be the same as the 77-78F and a head from them would be a direct swap.

[BlackMax]

Geeto-  thanks!  that pretty much spells it out.  I can buy a totally built (all brand new racing parts including valves) F head for $800.  I am still considering doing this, as it's 100% done, with nothing to worry about.    OR I could refurb. my existing F head (worn guides and really crusty) for:   

new guides:  $75

new exhaust  valves: $100

labor on the head:  $350 (guessing, to press in the guides and cut new seats, lap valves and general clean up)

So, by re-using my old head, I'd save maybe $275....but lose some time while the head is out, and also have some questions in terms of springs and retainers.  I wonder if saving $275 is wise or foolish when I could simply buy the new race head for $800, and bolt it on....then sell my F head.

[Geeto67]

what makes it a "Race head"? Does it have porting? Ti retainers? double sprung valve springs? Kibblewhite valves?

If you are talking about a head somebody ported and spent a lot of good money on then $800 is a bargain. If it is just a stock head with a rebuild and he is calling it a race head then maybe not so much. If you can see if the seller has the reciepts on the head rebuild so that you know for sure what it in there and who did it.

You won't get much for your junky old F head, I advise you keep it for now. If anything it shows any subsequent owner of your bike that you were careful about parts taken off the bike. Also god forbid the race head hangs a valve and eats itself - you then still have a good core you can then have rebuilt. Finally you can compare it to the race head to see what makes it different from the stock head. 

[MRieck]

25.00 a piece for F2 Valves?? You need to perform a valve job anytime you replace guides

[Porscheguy912]

While on the subject of F Heads I have a 78 F3 does anyone know if there was a factory recall on this problem or if there was a production fix I have around 25,000 miles and my motor shows no sign of a problem what should I be looking for?

Thanks
Bido

It is not a recall, just a design flaw and you should be looking for oil smoke out your exhaust. Honda increased the valve size in the 77-78F head but did nothing to shore up the valve guide. If the bike has been taken care of they start to wear out at about 30K miles. If the bike was beat up, less miles. The solution is to have new guides fitted by a machine shop. Dynoman sells the guides at dynoman.net

Blackmax - the 78 F head has a larger head volume and larger valves. To compensate for the larger combustion chamber volume the F pistons have dome shape to them to keep compression. If you put K pistons in an F engine you drop the engine's compression ratio significantly, if you put F pistons in a K engine you hit the valves. If you want to put a K head on your bike then I suggest you find K pistons, and I highly recommend (since you will have the engine apart anyway) that you bore it out to 836ccs to get back the power you will lose in switching to a more restrictive K head.

Also you are going to need a 1973+ K head if you decide to do this as the oiling passages in the cylinder and the head changed in 1973.

If all that was wrong with the head was the guides, buy the guides and have a machine shop install them, should only be a couple of hundred and not the $1200 you were quoted. At Dynoman a set of 4 cb750F2-3 valves is $30 (you need a set for intake and a set for exhaust so figure $60). You can probably reuse your springs and retainers (provided none are cracked), but if you wanted new springs it is only $65 for a full set. The guides are $70. Titanium retainers are $200 but as long as yours are not cracked you can reuse the ones you have. None of this is hard work and the only thing intimidating might be lapping the valves (easy).

edit: I should clarify - when I say K head I mean 1969 to 1976. The 77-78K motors are supposed to be the same as the 77-78F and a head from them would be a direct swap.

How Does One Go about noting the differences in the later K heads and the Earlier?
I'm going to a swap meet soon and I think i'm going to look to do this swap on my F3 Too. Anyone have a picture? Or is the some number stamped somewhere?

Will My Black F Valve Cover Fit The New K head?

Also, What kind of Coating (Paint, Powder) Will match up the nicest and not chip? I want my new K head to be Black Baby!

[BlackMax]

what makes it a "Race head"? Does it have porting? Ti retainers? double sprung valve springs? Kibblewhite valves?

If you are talking about a head somebody ported and spent a lot of good money on then $800 is a bargain. If it is just a stock head with a rebuild and he is calling it a race head then maybe not so much. If you can see if the seller has the reciepts on the head rebuild so that you know for sure what it in there and who did it.

You won't get much for your junky old F head, I advise you keep it for now. If anything it shows any subsequent owner of your bike that you were careful about parts taken off the bike. Also god forbid the race head hangs a valve and eats itself - you then still have a good core you can then have rebuilt. Finally you can compare it to the race head to see what makes it different from the stock head. 

Okay- here's the actual setup for the head I'm considering buying and bolting on. It's not a "race" head, in terms of taking it to the tack and competing.  It just has high-end parts, that would allow for as much cam as the pistons will allow (I'm running stock).

Valves:  Dynoman standard F2 valves (all 8 are new)
Springs:  Dynoman racing spring
Retainers:  Ti
Guides:  Dynoman new
It's not really P&P, but rather cleaned up.

So, for $800, I'd say it's a reasonable deal (for my purposes), considering all the work I'd have to do to mine, in order to make it "useable" (new guides, cut seats, clean up, maybe new valves).  In "useable" condition, I estimate I'd save about $250 or so (depending what the shop charges), but lose time while it's out and also I'd have those questions about the parts I didn't replace.   I have a feeling that the $800 head is the smarter route.   If I sell my existing F head,  I'd make a little money selling my F head (the valves alone must be worth something), PLUS I'd save money from my mechanic, as he wouldn't have to do much of anything further, just bolt the head back on and put the motor back in.

What's your vote?   New head or refurb. mine?

[NitroHunter]

I've run K heads on an F-2 block with out issues, the K just doesn't utilise the front oil drainback provisions. Same goes for F-2 head on a K block, oil simply builds up and pours back thru the cam chain opening.
 
Piston valve pockets are another issue - I'd HIGHLY reccommend to clay check clearance before final assembly when doing these swaps.
Don't ask me how I know this...


I had to check, the '78 service manual is giving conflicting info (pg 6-12)... but it appears that 77-78 K's use standard K sized 32mm intake /29mm exhaust valves.

[Clyde]

The 77K and 78K motors are basically the same as the 76F1 not the later F2/F3.
They use a slightly higher compression ratio piston(9.2 vs 9.0) than the earlier K motors, but use the smaller valves of the 69-76 K

[Jerry Rxman Griffin aka MuthaF'er]

PorscheGuy,

The 77/78 K head is a 75/76 F head. All 4 are -392 heads, the -392 is the model designation for the 75/76 750F. You will find "392" cast into the top side of the top fin intake side between cylinder #2 & 3 closest to 3. My K8 -392 head also has K8 hand stamped next to the 392. This K8 designation alerts you to different keepers, same valves with different grooves for the different keepers, and slightly stronger springs.   

[Jerry Rxman Griffin aka MuthaF'er]

Head alert,

EBay. Rebuilt, ported head. Chambers look hemi'd. Possibly F2/F3 or at least K/F0/F1. Description says intakes 33.9. Exhausts are larger too. If his measurement is correct it is either a F2/F3 or a K rebuilt with Kibblewhite 34mm head/5mm stems. Starting bid $299.99 I believe.

Good luck. Best man wins. Er, guy with biggest bank account.

I'd grab it if I hadn't already had Mike do 1 large valve head and had Mike grab the 34mm Kibblewhite kit for one I'm sending this week.

[BlackMax]

Nice, but I just bought an F2 head and cam combo, advertised as having a nice tight valve-guide fit for a great price (thanks to a fellow SOHC board member).  Yea, I could have spent another $grand$ on a nicely done head and cam, and maybe got another 15 hp........above 7000 RPM.  At all other engine speeds, maybe lost ground.  Since this isn't a street fighter, I guess I don't really care.

 My only concern with a used F2 head is that they all (apparently) WILL destroy the guides, even if they look okay today.  So, this may be a temporary fix.  However, what's "temporary"?  If it's 10k miles, that's probably 5 or more years.  By then, maybe I'll have built up the head I removed in the first place.

Thanks for all the help guys.  With your input, I think I've got it figured out....finally.

[Porscheguy912]

PorscheGuy,

The 77/78 K head is a 75/76 F head. All 4 are -392 heads, the -392 is the model designation for the 75/76 750F. You will find "392" cast into the top side of the top fin intake side between cylinder #2 & 3 closest to 3. My K8 -392 head also has K8 hand stamped next to the 392. This K8 designation alerts you to different keepers, same valves with different grooves for the different keepers, and slightly stronger springs.   

Awesome. Thanks for the info Jerry.
I didn't mean to hijack the thread, but it seemed to be relevant.
So does this mean that the 392 heads are a direct bolt on for a F2/F3 Cylinder Block or is there machining to do to get the oil galleries to line up? Also K Pistons to follow the 392 K head? Or can one use the 75-76F pistons since the 392 is also early F's Head?

I'm Prob over thinking this. 

[Silverback]

Newb here! What a great site! I am just realizing a project I have had in my head for a long time and just purchased a project bike (1978F). I will be rebuilding the motor, which is in pieces. There is no head assembly included with my purchase.

I have been reading all of the info regarding putting the K head on the 1978F cylinders and I am a bit confused. There is some conflicting info. I have a 1978F motor up to the cylinders and a 1977K head that I was going to use.

1. I have read that the oil ports are the same, and that they are different on the 78F and 77/78K head.
What is the definitive answer? Also, is the 77A head different than the 77K head?
2. Are the 77K heads domed to accept the higher compression pistons found in the 78F? I have read through many posts and have read that they are, and that they are not. Again, a definitive answer would be nice.
3. I will be installing a big bore kit and have read that the big bore pistons are different for the 78F and 77K set-up.
If you use the K big bore pistons, the compression would not be sufficient for an F head (That's what I read). This refers back to question 2. Are the 77K heads made to accommodate the domed F pistons or the earlier K pistons?

I am about to order tons of parts for my engine rebuild (valves, valves springs, retainers, guides, cam, adj. cam sprocket and pistons) and don't want to get the wrong pistons. I have all new gaskets. 

Thanks in advance for any info.

 

[Jerry Rxman Griffin aka MuthaF'er]

(1) The 77/78 F (-410) head will not properly fit any cylinders other than 77/78 F so if you use a K head use K cylinders. The 77 CB750A uses a -392 head. The -392 head originated with the 75 F. It's an updated K head. Same head as 75/76 F and 77/78 K which will fit any 750 EXCEPT the 77/78 F.

(2) 77K (-392)head is not domed enough for 77/78 F pistons. The -392 head has better combustion chamber design than it's predecessors.

(3) Depends on which application the big bore kit you use is intended for. If it's intended for a 77/78 F and you use a 77/78 F head, you're fine. These kits are rare. If you use the standard K big bore kit with K head and cylinders you're fine. Don't use the K big bore kit with a 77/78 F head (which you don't have anyway). Most K big bore kits used in conjunction with K top ends will give you 10.25:1 or 10:5:1.

(2) the 77 K head will accommodate any pistons properly except the 77/78 F pistons.

A little history at this point. For the 75 F Honda decided it needed to increase the performance once again after seriously detuning the engines starting in about 73. They redesigned the combustion chamber by removing the "shrouding" around the spark plug hole and valves. Kinda like a more hemi look. Probably better air flow characteristics. They also bumped the compression ratio from 9.0:1 to 9.2:1. Possibly from the chamber &/or pistons. Plus a much better cam. Honda has had 2 different piston top designs. My 75 F (bought new so I know they were stock) had the "newer" ones while my 78 K engine (parts engine) had the old style. And these 2 engines were supposed to be identical.

Perhaps be more specific about which kit you are using. Put up a link. Don't know that you'd need new valves for a K head unless you are using larger size. Definitely stronger springs. Get a high performance cam! If you are putting that kind of $$ into the top end why not talk to MRieck about spiffing up the head? 

 

[Silverback]

Thanks for the info!

I will be using the 10.5 J&E piston kit listed on this page:
http://www.dynoman.net/engine/pistons/j-and-e.html
I have J&E pistons in my Aprilia, and have been very happy with them.

...or possibly the 850 kit offered by CycleX. It is half the price which makes me a little weary. You know, you usually get what you pay for. Any opinions?

I do have the cam from the 78F head, and even the valve train. I also have the F pistons. I will be getting a performance cam. Not sure how hot at this time, but I prefer low to mid performance over high RPM performance. This will be ridden primarily on the road. I was mainly getting new valves/guides to start with a fresh top-end. I am having a professional engine builder freshen up the head for me and bore my cylinders. He (Mike Wells) has built numerous reliable racing engines over the years and is 5 minutes down the road from me.

OK, so off to ebay to buy that K cylinder.

[Jerry Rxman Griffin aka MuthaF'er]

I think you're OK at this point. The F cam and springs would be better than the K stuff but not as good as say the Webcam 63B from Dynoman's link or a 125/75 (125/70) Megacycle. Definitely do new HP springs and possibly Ti retainers if cash permits. The stock springs in my 75 F and a missed 2nd gear power shift is what got me into all this. Broken valve obliterated my sweet almost new engine. Those stock springs aren't good for 11K or 12K. Floated that damn valve right into the piston. Good and bad. Many $$$ and years later I am MUCH better informed and educated.

Could be that the CycleX kit would work as well. Don't know. What do you expect out of it?!That would buy the cam with the difference. Few guys here have used it. Not likely you can find any high mileage testimonials yet. I'm not a high mileage guy on a high performance engine. I'm a high performance guy on a high performance engine guy. I just don't think the 2 should coexist. I ride my semi high performance Fat Boy for high mileage. I am in the process of slowly building 2 $5K 750 engines, both high perf. A 900 and a 1000. Neither will get high miles but I'd bet they'll run like a scalded ass monkey! 

The K cylinder is the best and easiest way out. Oh yeah, get a set of heavy duty cylinder studs from APE!! That will delay the inevitable oil leak at the head gasket.

Happy engine build. Only your bank account will regret it.    Next we'll deal with your carbs. I wish mine was that simple now. I did it once with my 812 Yoshi kit. I've gone FN crazy now that I can afford it.

Jerry