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Discussion Starter #1
I’m looking to lap in a ct70 head I bought to ensure a perfect seal will I have it apart....will probably do the same on my z50 when I pull it apart soon. Wondering if you guys have found a tool that work with these small heads. I have a cranking one but the suction end is way too big for these valves.


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I just put a piece of poly or rubber hose on the stem, then spin it between my hands to spin the valve. Works great.
 

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Know a lot of guys that do this on Honda car engines. they put compound on the valve, a plastic tube on the valve stem then a bolt in the other end. Chuck the bolt up in a cordless drill and spin away.
 

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Discussion Starter #4
Tube on the stem, brilliant! Thank you.


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Discussion Starter #5
Ok, so last night I used the tube method to lap the valves in. Again, great suggestion. I really gave them a good lapping, taking them out, inspecting, new compound and relapping. Thought I had it. Installed and did a leak down and still leaking. SOOO, tear it all down again, lap some more. Ran out of time to test....but something tells me I’ll get the same result.

So, I have the rocker arm gap set correctly. Motor is at TDC and held in place with a breaker bar. The rocker arms wiggle so I know they are not opening the valves. Lapped valves in with permatex water compound I believe. Thoroughly cleaned after. Timing is set correctly.

Is there something I am missing here? Is there something else needed to ensure a good valve seat seal? Thinking, I had my leak down set low, something like 30psi actually. Did this as I was trying to hold the flywheel with hand from spinning on first go. Would imagine more pressure would not help. I didn’t even look at the difference on the gauges as the leak is instantly heard.


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Are you hearing air coming from carburetor or exhaust pipe? That will tell you which valve is leaking. If its not coming from either of those pull the dipstick and can you hear it from there? Then it’s the piston rings.

also when you say it’s at TDC that means the “T” is at the mark on the case and the “O” on the cam lines up with the mark?
 

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Discussion Starter #7
Yes to all. Well versed in using one...but never on a tiny engine like this. I mean, to do work on classic minis but... it’s great having to run around to the back of the car and get your ear up to the exhaust. :)

So the motor is on the bench and has not been run nor has any intake or exhaust attached. The air is very clearly rushing out the intake. You can hear it and feel...even make fun music when you cover/uncover with hand. Ha!


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When/after you lap the valve in, look at the valve, AND the seat. You should see the grey line where they have sanded each other. It should be even and consistent all around the mating surfaces of both parts. If there are pits, or any inconsistencies...that's likely your leak.
Most likely you need a valve job. New valve(s) and seats cut...the whole job complete and correct.
Nothing better than a newly rebuilt cylinder head... IMO.
 
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