K4 and K6 engine compatibility?

[finnblade]

First time poster and a new owner of a Honda CB750K6.

I recently bought a K6 CB750 SOHC and the tranny cases have a huge hole due to a snapped chain. I came across a K3/4 engine and I am wondering if it would bolt up nicely with the K6 frame, carbs and exhaust. The motors seem identical as far as I can tell, but what's the truth? Will the K4 engine bolt up with no issues?

Thanks,

-Tom-

[TOOLS1]

Welcome to the forum. Yes the engines are the same, and will bolt right in any K, or F frame. The A was a little different.
TOOLS

[finnblade]

Thanks Mate,

Both are manual models, so I think I'll just swap the engine and take a winter project to see if the original engine can be repaired. It's got a quarter size hole just in front of the front sprocket.

br,

-Tom

[rrgunslinger]

Cool keep us in the loop. With pictures. 

Harvey

On Jun 2, 2012 12:59 PM, "finnblade [via Honda CB750'S]" <[hidden email]> wrote:

Thanks Mate,

Both are manual models, so I think I'll just swap the engine and take a winter project to see if the original engine can be repaired. It's got a quarter size hole just in front of the front sprocket.

br,

-Tom

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NAML

[Lucky 1]

I hope you do not throw away those cases with the hole in them.
They are easily repairable.
Please give them to someone.

[finnblade]

Lucky 1,

Don't worry. I know what a pain in the donkey it can be to find parts for older cars and bikes. I've restored couple of old SAAB cars and the biggest sin is to bin some repairable parts.

I think I'll pull the old engine apart and try to fix the cases.

And I'll try to post some pics of the swap. It looks like a bit of an squeeze though. Not much room and the engine weighs a ton. If you guys (or gals?) have any good pointers on the swap, please feel free to share them. It will be appreciated.

-tom-

[shinyribs]

Whew,I'm glad to hear you say that! I was thinking that I would hate to see those cases get tossed,too. I am sure they could be welded,but since that area really does not have a lot of stress on it,is epoxy an option? I'm sure it would not the the prettiest cure, but if you are just looking to seal the oil in,it might just work.   Either way,good luck with your project(s)!

[finnblade]

I thought about some epoxy-JBweld-solution, but since I came across this other engine and the hole is not that accessible, why not drop the new engine in and try to fix it properly. It would be a perfect winter project and if the fix does not work, then I still got a working K6.

And I've got a few CBR's (1000RR & 600F) to ride around with during the summer so I haven't got the time or motivation to spend much time in the garage...

-tom-

[finnblade]

Time for an update: I just bought a K6 engine and a set of carbs. Now only thing is to pull the old engine out and drop the new motor in. Any advice for a CB newbie what's the right way to go about such task. It seems to be a pretty tight fit..


-tom-

[TOOLS1]

Strip the bike, and lay it on it's right side. Then lift the frame off the engine.
TOOLS

[shinyribs]

TOOLS1 wrote

Strip the bike, and lay it on it's right side. Then lift the frame off the engine.
TOOLS

Works for me!   http://honda-cb750-s.456789.n3.nabble.com/One-man-engine-removal-in-ten-minutes-td3248305.html

[finnblade]

Allrighty,

I got the engine off the frame with no problems, and the new motor went in as smoothly as the old one came out.

I've put something like 200 miles on the bike now and I'm surprised how nice it is to ride. I've tried a -77 Norton Commando before, and the Honda is totally different. Feels much more modern.

Now I only have to pull the old engine apart and get the cases welded and rebuild the engine. This looks like a nice winter project.

-tom-

[pat english]

i got a good set of cases for ya..$60 shipped..let me know.

[LukeM]

Congrats!  Nicely done.  I would expect a bit of a difference between the Commando and the CB.  Norton is more like a WW2 propellor plane, the Honda more like a Korean war jet.  Both do the job, but in different ways.

Good luck with the rebuild.  Should be a fun one.  Be careful out there.
Luke M

[finnblade]

Hi Pat,

Thanks for the offer. I bet shipping costs to Finland would be a bit too expensive.

-tom-

[finnblade]

I just got around to dismantling the old engine. It seems major part of the damage is isolated to the lower cranckcase. Upper cranckcase has a small crack, easily welded.

Are the cranckcases machined as a pair, or can I just source up a new lower case and put the engine back together? I think the problem would be with the cranckshaft bearing clearances...

And why would I want to keep the original cranckcases I hear you ask. Here in my neck of the woods, there's a separate registry for restored bikes and cars, and they are a bit picky about changing the matching numbers engine with something else. As you all know, the engine number is located on the upper part of the cranckcase.

Thanks,

-tom-

[TOOLS1]

The engine cases are machined as a pair, and can not be switched. If, you take them apart, you can see they are marked from the factory. Also the engine numbers, and frame numbers never matched, so I don't know why that would be a problem.
TOOLS