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Discussion Starter #1
New to the forum, thanks for looking!

I've got a 1973 Z50 that I just installed a new 88cc big bore kit. Everything went relatively smoothly with that. Now it looks like my intake valve is bent slightly so I have been looking for a new valve or refresh kit. The valve dimensions are 22mm, 66mm length, and 5.5mm diameter. After looking online for the stock measurements for a 1973 Z50 I think what is currently on my bike isnt stock. I think I found the stock dimensions to be 18mm/68mm/? diameter. Can anyone confirm or know what may be up with this?

Overview:
1973 Z50
What I think is stock intake valve... 18mm/68mm/?diameter
What is on bike now intake valve... 22mm/66mm/5.5mm

Anyone have a good idea on what may have happened or a good path to move forward in getting this sorted out?

I see the 70cc engines have a stock 23mm/20mm/5mm. I feel like i would be able to work with that if I lap the valves except the mismatch diameter might be an issues.

Thanks for the help!

-Scott
 

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Discussion Starter #3
Can you post up some pics of the head, to see if there are any Honda-type numbers on it?
Sure thing. Here you go. Let me know if there are any other specific ones you need. Looks like “C3” is on there a few times.
 

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According to the Clymer Manual, the standard valve stem OD for the 50, 70, and 90cc engines should be 5.455-5.465mm for the intake.

Is that a beveled edge between the combustion chamber area and the flat surface of the head, or is it all flat until you get to the combustion chamber?
 

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Discussion Starter #5
According to the Clymer Manual, the standard valve stem OD for the 50, 70, and 90cc engines should be 5.455-5.465mm for the intake.

Is that a beveled edge between the combustion chamber area and the flat surface of the head, or is it all flat until you get to the combustion chamber?
5.46 is what it is around now.
The edge is not beveled. It is flat until the combustion chamber.

sorry, where are those tech specs? I’m interested to see what the standard is of the valve head diameter and length.
I guess I’m just trying to mainly find a replacement that matched what I currently have. I haven’t been able to find one as of yet. Maybe I don’t know the main parts places to check. I did email TBParts to get their valve dimensions because they weren’t listed.
 

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The Clymer Manual does not list the valve face diameters, only the above valve stem information.

I don't think that it is a stock OEM Honda head, the 22mm valve face seems large for a 50cc engine, and more apt to be found on a stock 70cc head, which would normally have the beveled edge like I asked about above.

Do the valves have a recessed face with a machined groove in the center, like pictured below?

270880
 

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Discussion Starter #7
The Clymer Manual does not list the valve face diameters, only the above valve stem information.

I don't think that it is a stock OEM Honda head, the 22mm valve face seems large for a 50cc engine, and more apt to be found on a stock 70cc head, which would normally have the beveled edge like I asked about above.

Do the valves have a recessed face with a machined groove in the center, like pictured below?

View attachment 270880
yeah, with the size of the valves I was kinda thinking the same thing.
Yes, there is def a notch in the intake and it kinda looks like one on the exhaust side too. Too much hard carbon to see on the exhaust side.

I hope this big bore kit will work with the 70 head and not the 50.

I found this valve on eBay. Looks like the right shaft diameter. Think being a 23 and not a 22 will be an issue? It does look like there is room on each side of the 22mm old valve.
 

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Discussion Starter #8
Here is the pic showing the room to spare.

the big springs are 27.4mm. Not sure if I should go ahead and get new ones of those too while I’m in there. Everything else seems to be in descent shape. The exhaust side valve seat seems good. I got new ones in a gasket set but they are the larger ones. No seat on the intake side. Exhaust valve seems to seating itself well so no need to change that one I don’t think.
 

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Discussion Starter #10
Chances are the extra mm won't be an issue, but it probably wouldn't hurt to clay the piston to make sure it doesn't hit the valve.
Thanks for your help with this. I am going to give the new single valve a go. I will try out the clay too so I don't blow anything up.
 

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Discussion Starter #11
New valve is in and it seats nicely. Thanks again for your help.

But now, the flywheel doesn’t seem to always spin well/much. After reading the forums some I think it is either a worn out clutch or and/or the fact that I threw synthetic oil in it the other day because I could wait to try and kick it over. Does that sound right? I’ve got some 4-stoke oil on the way here tomorrow.
 

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With the 88 kit you now have an increase in compression, and the clutch may be slipping a bit more now with that increase in resistance. I'll assume you're experiencing the flywheel not turning over when you are kicking it, and you've found that if you ease through the kick downstroke, it turns over for you, correct?
 

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Discussion Starter #13
With the 88 kit you now have an increase in compression, and the clutch may be slipping a bit more now with that increase in resistance. I'll assume you're experiencing the flywheel not turning over when you are kicking it, and you've found that if you ease through the kick downstroke, it turns over for you, correct?
kinda. I thought it was turning more and quicker when I was kicking it slowly. I just went back out and it wasn’t turning much at all if I was kicking it slowly.
 
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