Topic: can I put 750 78 K head on 78 f cylinder block?

[Serge]

can I put an 78 K head on 78 f cylinder block? do they have the same covers as well ? found my oil tank empty again..

[mazingerzeca]

I was trying to do the same, because I waste a lot of oil too. Is there any specific problem on F2 head that makes them waste oil or with a good seat job and new guides and so on it can be solved?

[jtb]

Pistons are different from K model to F2.  F2 pistons are domed, and have no relief cuts for the valves.  Other than that, I don't know whether it would work.  Mike?

[Serge]

I was trying to do the same, because I waste a lot of oil too. Is there any specific problem on F2 head that makes them waste oil or with a good seat job and new guides and so on it can be solved?

apparently the valves on f2 are larger than in previous SOHC. The fact is that you can not get them from honda anymore, so rebuild gets really expensive with $60 per after market valves .. adding guides and and shop charges to install them it does not make sense other than for sentimental reasons, there are also these oil passages which are blocked on F2 on the base of cylinder block and in my case i sealed them from the top after putting the block and it looks like oil is getting out at highway speed... 

it seems one can get complete used heads for sohc 750 on e-bay pretty cheap, I was wondering which i can use one my block with my camshaft ..

[jtb]

You can probably use any head with the camshaft, but if you do switch to a K head, you'll need to use K pistons, too.  For that much trouble, get a K head and an 836 kit.

[Serge]

Correction: I actually have F3 head, (cb750E3107775) though i guess it makes little difference..

[jtb]

Still has the bigger valves and domed pistons.

[Hockers Choppers]

Make sure you get a k head and cylinder! The F cylinder doesn't return oil like the k. You need to replace them as a set! I have an F3 and am going thru the same thing. F heads have a bad valve retainer design. If you go with the K head you can get better aftermarket parts, and the big bore kits, and cams are designed to work with them. If you want to run the F head you will have to spend big $$s.
Jesse

[mazingerzeca]

Make sure you get a k head and cylinder! The F cylinder doesn't return oil like the k. You need to replace them as a set! I have an F3 and am going thru the same thing. F heads have a bad valve retainer design. If you go with the K head you can get better aftermarket parts, and the big bore kits, and cams are designed to work with them. If you want to run the F head you will have to spend big $$s.
Jesse

I'm in the same trouble too. But, if you use K7 pistons and a K7 head, can you use the F2 camshaft or is needed a K7 camshaft? They are not the same? (I have a F2 engine and a K7 head without camshaft and holders, so I would have to use them from F2's head)
Thank you.

[jtb]

The camshaft holder set is the same part number for the K7 and the F2.  The cams are different numbers, but probably would change out ok.

[scondon]

   If you want to use a different head on your F3 motor then the easiest solution is to swap out everything between the base gasket and the valve cover with the exception of the cam tower/rocker assembly.Head,cylinder,pistons and cam from a donor motor. You CAN use the F3 cylinder block but you will have to machine troughs(channels) in the base of the cylinder to connect the outer oil drains with the inner(unused) oil drains. I'd only use the F3 cylinder if you're planning on boring them for oversize pistons.

   The absolute easiest solution is to get a K7-K8 motor that runs and put it in your bike. Then you can take all the time you want rebuilding the F3 engine.

[MRieck]

   If you want to use a different head on your F3 motor then the easiest solution is to swap out everything between the base gasket and the valve cover with the exception of the cam tower/rocker assembly.Head,cylinder,pistons and cam from a donor motor. You CAN use the F3 cylinder block but you will have to machine troughs(channels) in the base of the cylinder to connect the outer oil drains with the inner(unused) oil drains. I'd only use the F3 cylinder if you're planning on boring them for oversize pistons.

   The absolute easiest solution is to get a K7-K8 motor that runs and put it in your bike. Then you can take all the time you want rebuilding the F3 engine.

Yes ...I just told this to Dusthawk, don't reinvent the wheel here............

[Serge]

   If you want to use a different head on your F3 motor then the easiest solution is to swap out everything between the base gasket and the valve cover with the exception of the cam tower/rocker assembly.Head,cylinder,pistons and cam from a donor motor. You CAN use the F3 cylinder block but you will have to machine troughs(channels) in the base of the cylinder to connect the outer oil drains with the inner(unused) oil drains. I'd only use the F3 cylinder if you're planning on boring them for oversize pistons.

   The absolute easiest solution is to get a K7-K8 motor that runs and put it in your bike. Then you can take all the time you want rebuilding the F3 engine.

Thanks Sean !  I guess I will take this route. What year bikes I should look for K7-K8 donor motor?  - Serge

[mazingerzeca]

   If you want to use a different head on your F3 motor then the easiest solution is to swap out everything between the base gasket and the valve cover with the exception of the cam tower/rocker assembly.Head,cylinder,pistons and cam from a donor motor. You CAN use the F3 cylinder block but you will have to machine troughs(channels) in the base of the cylinder to connect the outer oil drains with the inner(unused) oil drains. I'd only use the F3 cylinder if you're planning on boring them for oversize pistons.

   The absolute easiest solution is to get a K7-K8 motor that runs and put it in your bike. Then you can take all the time you want rebuilding the F3 engine.

Hello Scondon. I would need to use the F2 cylinders, because I don't have any donor engine. What should I do exactly to connect the outer oil drains with inner oil drains? You mean something like a channel to make both holes go to the same place? Thank you.

[scondon]

If you want to use a different head on your F3 motor then the easiest solution is to swap out everything between the base gasket and the valve cover with the exception of the cam tower/rocker assembly.Head,cylinder,pistons and cam from a donor motor. You CAN use the F3 cylinder block but you will have to machine troughs(channels) in the base of the cylinder to connect the outer oil drains with the inner(unused) oil drains. I'd only use the F3 cylinder if you're planning on boring them for oversize pistons.

 The absolute easiest solution is to get a K7-K8 motor that runs and put it in your bike. Then you can take all the time you want rebuilding the F3 engine.

Thanks Sean ! I guess I will take this route. What year bikes I should look for K7-K8 donor motor? - Serge

  That would be the 77-78 750K engines. I only mentioned these years 'cause I think they also ran the 630 chain.You can put ANY year SOHC 750 engine in your bike. The 77-78 K and 75-76 F's all have exhaust stud mounts in the head so there wouldn't be any trouble fitting your exhaust to any of these engines(maybe 75-76 K bikes too).

If you want to use a different head on your F3 motor then the easiest solution is to swap out everything between the base gasket and the valve cover with the exception of the cam tower/rocker assembly.Head,cylinder,pistons and cam from a donor motor. You CAN use the F3 cylinder block but you will have to machine troughs(channels) in the base of the cylinder to connect the outer oil drains with the inner(unused) oil drains. I'd only use the F3 cylinder if you're planning on boring them for oversize pistons.

 The absolute easiest solution is to get a K7-K8 motor that runs and put it in your bike. Then you can take all the time you want rebuilding the F3 engine.

Hello Scondon. I would need to use the F2 cylinders, because I don't have any donor engine. What should I do exactly to connect the outer oil drains with inner oil drains? You mean something like a channel to make both holes go to the same place? Thank you.

  I cut a 5mm wide by 5mm deep channel into the aluminum base surrounding the blocked oil drains. This pic shows the area needing to be cut.  Since the F2 pistons won't work with any other head but the F2 head I'm assuming you're going to bore these cylinders for oversize pistons?

[mazingerzeca]

  I cut a 5mm wide by 5mm deep channel into the aluminum base surrounding the blocked oil drains. This pic shows the area needing to be cut.  Since the F2 pistons won't work with any other head but the F2 head I'm assuming you're going to bore these cylinders for oversize pistons?

Hello. Yes, I will use oversize pistons (not 836, 0,25 rebore) and I will use K7 pistons, much more available than F2 oversize pistons. I will also use a K7 head. This plan to "convert" F2 engine to K7 is to have a more "normal" engine. I need to use F2 cylinders because I don't have access to K7 cylinders (In Spain they are not very common). So, in the picture you posted, I understand that is the bottom side, isn't it? and the holes to be drilled are the ones on the left? Sorry for all that questions, but I don't have the cylinders dismantled and I can't see them now...
Thanks for your help.

[scondon]

  Yes, the pic shows the bottom side of the cylinders. There will be four areas that will need to be cut. Rather than drilling you will want to cut a trench(channel,passage?) into the half circle area to connect the closed drain to the larger open one. I thought I had a pic of the cuts I made and will look again when I get home tonight.

   The oil drains from each of these two passages and flows down a very narrow channel cut into the crankcase. When you get the engine apart and take a look at how the oil drains from the head back to the crankcase then it will make much more sense as to how and why the drains need to be joined.

   As far as your earlier question about whether your F2 cam will work. I'd be inclined to say no, but I don't know for certain. It does have a different lobe profile and higher lift than the other cams so it may not work with the K7 head and pistons. Webcam makes some pretty inexpensive cams here in the states and one can be bought with just mild performance increase to be used with stock head and pistons(K models and 75-76). Dynoman in Texas is a good supplier and might ship to Spain. Their website is listed in the FAQ

[mazingerzeca]

  Yes, the pic shows the bottom side of the cylinders. There will be four areas that will need to be cut. Rather than drilling you will want to cut a trench(channel,passage?) into the half circle area to connect the closed drain to the larger open one. I thought I had a pic of the cuts I made and will look again when I get home tonight.

   The oil drains from each of these two passages and flows down a very narrow channel cut into the crankcase. When you get the engine apart and take a look at how the oil drains from the head back to the crankcase then it will make much more sense as to how and why the drains need to be joined.

Maybe you mean doing something similar to this (the picture is from Honda K7-F2 manual, but this picture must be from K7 cylinders). So, I will do this F2 to K7 conversion.

This is what I will do.
- Drill the bottom of the F2 cylinder
- Install 0.25 overbore pistons on rebored F2 cylinder
- Install K7 head with new camshaft from Dynoman
- Install knock pins and stud gaskets for the 4 K7 head oil passages

I think I don't forget anything for this Frankenstengine. Thanks again.

[scondon]

Sounds like you've researched your project well and have all the bases covered

   Yes, your pic shows the K7 cylinder base and that is where you want to join the passages. You don't have to remove much material at all. In other words, you don't have to make it look just like the K7 cylinders.

 After the oil drains down the rear cylinder holes it must travel down a slot cut into the crankcase which is no more than 4mm wide and 2-3mm deep, I'm guessing. Therefore your cut doesn't have to be any bigger than that. 5mm x 5mm is plenty (I think mine were cut 4mm x 3mm).