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Discussion Starter · #1 ·
Hi 450 nuts
I just joined the group and I used to be on the Yahoo DOHC group years ago.

I bent a valve on the 4 speed top end and I have a good condition K1 top end that just needs rings.
I figure it's easier to get 5 speed parts (pistons, rings, valves, etc) and this bike doesn't need to be original so just swapping the whole thing to 5 speed is the way to go. I know I could just replace the whole engine but I want to resurrect these cases as they have some history to me.

Before I spend a lot of time measuring and playing with it I figured I'd ask if anyone could tell me in detail if a 5 speed top end (pistons, cylinders and head) will work on a 4 speed bottom end? I know the 4 speed pistons wouldn't work with just a 5 speed head put on due to the dome shapes but if I take the whole kit and bolt it on are there any issues?

I know the bore and stroke is the same, the camchain, etc will work. I guess the only doubt I have is if the balance will be off due to different weight pistons. You'd encounter the same thing with oversize pistons and most people don't rebalance but I figure I'd ask if there was anything I am missing here.

I searched all 26 pages of this forum and found a lot of great info but nothing covering this



thanks in advance
 

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Sensei
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Hello Augustiron...I do remember you from the MSN site.....
I have never personally used a 5-speed top on a 4-speed lower, but can see no reason it wouldn't work..... However, I would check the 'deck' height of both cylinder/piston assemblies to be sure 5-speed pistons rise to an appropriate height when on the 4-speed crank/con rods (I seem to remember a different wrist pin to crown height on the 4 and 5 speed pistons as well as the difference in dome shape).
Please let us know your findings....... Steve
 

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Many moons ago I fitted a K1 top end to a KO bottom end.Its still stood in the bottom of my garage as pic. here in the UK.It is a four speed bottom end although it does have a 5 speed engine cover on it.I remember the cam chain needed to be one link longer than the original K0. but everything else fitted OK.The inlet valves are larger on the K1 and so K1 carbs were fitted.However this was in my early days and I never got it to run right and as I had a spare KO engine it was easier to fit that. Its always intrigued me why I never got it to run properly and maybe oneday I'll try it again or probably its better just stood there for if ever I need the spares.Regards Chris.
 

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Discussion Starter · #4 ·
After much tinkering I am going to try an experiment just to see if it works.

4 speed head, cylinders and lower end with modified 5 speed pistons.

Roughly measuring the 5 speed piston versus the 4 speed with the limited tools I have, the pin to top distance seems the same and it is the squish area at the outer diameter of the top of the piston that interferes with the 4 speed head's combustion chamber because the pistons have a slightly different shape out there.
My plan is to take a set of old 5 speed pistons and I am going to machine down that outer area to match the 4 speed piston. I am going to leave the dome intact to begin with and clearance it for the valves if needed.
I am going to dryfit and clay check the rotating assembly clearances this way. If it works, I will be keeping the nicer cast 4 speed head with the smaller exhaust studs but I will have higher compression as the domes on the 5 speed are larger than the rounded domes on the 4 speed pistons. I don't have any tools to measure volumes and such so I will have no idea what this will do to the compression except to try it.

Why you ask? because I like doing weird things like that, I have several spare engines to play with and I want to try something new.

any thoughts? :lol:

BTW, this project is a back from the grave custom that had no chance of being restored, so please forgive my blasphemy of hacking a K0.
 

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Good luck - I do know that 4-speed pistons have a flatter "dome" on top, this might cause you problems.
But if you're using 5-speed pistons and head, seems like it might work.

And Chris - didn't the 4-speed have a different screw pattern on the clutch cover??
 

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Hi Bill,
4 speed clutch covers will fit onto 5 speed only difference is length of one of the top screws (Longer on KO's :D) and the rib near the kickstart not on later models.(See pics)All the holes are the correct positions.I only noticed recently on the other side cover the later models don't have the little "notch" on the casing(near my masking tape on grease nipple) again as pics. Regards Chris.
 

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Discussion Starter · #7 ·
I am going to try the 4 speed head and a 5 speed piston (bigger popup versus round dome of the 4 speed) plus it's readliy available as a replacement versus the 4 speed one.

I placed the 5 speed piston into the 4 speed head combustion chamber and the only thing that didn't fit the same as the 4 speed piston was the squish area I am referring to. About .25" around the outside diameter before the pop up starts seems to be the interference. I will check again but I think if I machine that I am good. we'll see.... :lol:
 
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