Topic: can I put a cb 750 F2 camshaft on a K7 head?

[mazingerzeca]

Hello.
Due to all these particularities of the F2 head (difficulty on finding new valves, bad valve seats and so on) I am going to use a K7 head on my F2 engine. But... will I have to buy a K7 camshaft, holders and rockers or can I use these from my F2 engine? (I only have the head, with the valves).
If not, which camshaft should I buy? webcam, megacycle, OEM?
Thank you.

[MRieck]

Hello.
Due to all these particularities of the F2 head (difficulty on finding new valves, bad valve seats and so on) I am going to use a K7 head on my F2 engine. But... will I have to buy a K7 camshaft, holders and rockers or can I use these from my F2 engine? (I only have the head, with the valves).
If not, which camshaft should I buy? webcam, megacycle, OEM?
Thank you.

You can use the F2 cam, towers etc. You don't have to change anything out

[bhkrause]

Hi, Mike. 

Don't the F2 heads have larger diameter carb intakes?  How do you seal the F2 carb boots to the smaller F7 intakes?

Bernie

[JohnG]

Bernie has a real good point. We need someone with a K7 head to put a caliper on both sides of the carbs to see what their OD is.  My recollection is also  that the F2 had bigger carbs (30 mm instead of 28? is that right?)  A few years ago I went to use the air filter housing from an F2 with an F1 and found the boots on the back end were too big.

While I dont have any quick fixes for bad valve seats, I know that Manely sells what are oversized valves for the earlier CB750s (stainless) which means they might work as standard sized valves for the F2 (especially if mated to new seats).  The Manley valves have the advantage of being grindable making them essentially lifetime purchases. I believe retail is about $25/valve.

So continuing along that path, does anyone have a source for replacement valve seats for CB750 heads?  With the secondary question of whether they would be the same for the F2s?

Since all this is on paper, we might as well investigate all possible solutions to his bad head...

           John
           '76 CB750F

[bhkrause]

Found another old email from Mike that said you'd have to use K pistons as well - the F pistons are domed and will up the compression too much.


See this post for mix'n'match capabilities.  Unfortunately doesn't talk about pistons and/or carb boots.

http://www.sohc4.us/forums/index.php?topic=369.0
bk

[mazingerzeca]

Thanks for fast replies.
As far as I know, the K7 and F2 have the same carburettors, so the intake boots should be the same.
Yes, I know that I will have to use k7 pistons, but anyway I need to rebore te cylinders and I won't find oversized F2 pistons. This is another reason to "convert" my F2 engine into a K7.

[Egil]

Don`t no if this link can help you on the road(way) of something   

Sorry this link is dead:
`http://www.partspartners.com/html/cb750.html`

[mick750F]

Hi, Mike. 

Don't the F2 heads have larger diameter carb intakes?  How do you seal the F2 carb boots to the smaller F7 intakes?

Bernie

   
   The carbs are the same size so you would use the K7/8 carb boots.

Mike

[eurban]

As mentioned, the K7/K8 & F2/F3 carbs are identical in dimensions so there is no problem with the carbs themselves.  But to clairfy, the F2/F3s have larger intake spiggots on the head so the intake rubbers are different.  Use rubbers from a K7,K8 to fit your K7 head . . . .

[bhkrause]

Ok, so here's a dumb question - is there a way to reduce the compression/interference of the F2 piston with the K head by using a thicker gasket, or would the gasket have to be too thick?  How much more domed is the F chamber?

Anyone have a cross sectional diagram of the F and K heads with measurements?

[JohnG]

If anyone has a good stash of Cycle Magazine from 1977, when the road test of the F2 was done, they compared the head/valves/pistons side by side with the F1 (which is the same as the K7) and had some good pictures.

[Bob Wessner]

If anyone has a good stash of Cycle Magazine from 1977, when the road test of the F2 was done, they compared the head/valves/pistons side by side with the F1 (which is the same as the K7) and had some good pictures.

Wow, wish my memory was that good. 

[JohnG]

Thank you...I wish it really was that good.

I found the road test of the F2 in a 1977 Cycle magazine. Someone had sent it to me in electronic form a couple of years ago.  No engine photos....

There was an article from that time frame (77-78) which focused on the inherent problems of that particular motor. Honda was trying to get a little more horsepower and had gone the high RPM route (the F2 had a 9500 red line vs  the 8500 of the F1) by using bigger valves and larger chambers. The bigger valves were also a little heavier and that combined with higher domes meant less reliability. So whatever the article was had the F2 components side by side with F1 components. I seem to recall the lift on the cam was a little more but I would have to verify that from the Honda shop manual.

                                              Not-So-Hot-Memory-Guy     

[mazingerzeca]

I seem to recall the lift on the cam was a little more but I would have to verify that from the Honda shop manual.

                                              Not-So-Hot-Memory-Guy     

Yes, that's what I'm affraid of. I searched for that on the Honda Shop manual, which is the same for K7 and F2, but they don't especify any difference between both camshafts. But in the Parts list the number of both is different in K7 and F2. It would be nice that someone who disassembled both heads could certify the difference between camshafts and their interchangeability.

[JohnG]

According to Mark Haycock's book on the CB750, the differences between the F1 and F2 motors were as follows (noting that the F1 engine and K7 are the same I believe):

"Bigger valves were fitted (inlets increased  from 32 to 34 mm and exhausts from 28 to 31 mm) and the camshaft was reprofiled to suit.  This was the hottest cam fitted to any CB750 with 225 degree opening and 0.5 mm extra lift for both intake and exhaust.  The valve springs were strengthened (from 27 N/mm to 35). This reduced valve bounce so the engine's redline was at 9500 RPM".

"The combustion chamber and port shapes in the head were altered, and the compression ratio reduced to 9.0:1".  (page 109)

A website I stumbled on said the combustion chamber volume was increased by 18% so if one used F2 pistons with an F1 or K head, the CR would be a little over 10:1.

I can get the F1 camshaft part # but don't have a fiche for the F2 but the middle part should no longer be -392-.

            John