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Hit Miss Valve Stem Clearance

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Tractor Guru

Joined: 12 Nov 2013
Posts: 23865

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PostPosted: Wed Jun 09, 2021 11:12 am    Post subject: Hit Miss Valve Stem Clearance Reply to specific post Reply with quote

Trying to get my Fairbanks Morse 3HP ZC back together.

Took the valves out, the seats and faces look great.

The stems have minimal wear, they check at .373 to .371.

But the guides are really loose. Checking with gauge pins I get .387 on the intake, .382 on the exhaust.

I have no idea what they are supposed to be. Is this acceptable for it being an open valve train?

It's just going to be a running display motor unless I can come up with something for it to run, still it's working days are over. Just want it to start easy and idle down.

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Tractor Expert

Joined: 21 Aug 2016
Posts: 2272

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PostPosted: Wed Jun 09, 2021 5:04 pm    Post subject: Re: Hit Miss Valve Stem Clearance Reply to specific post Reply with quote

That intake valve has to be pretty loose so suction will open it and that weak spring will help snap it shut on compression. I dont really think there is a factory spec, just loose enough to easily push in without any excessive side-to-side play. The mechanically actuated exhaust valve can be a bit tighter in the guide, I suppose.
Bush them and then ream them to size- trial and error until you get a fit that feels right- and dont forget to drill out the oil weep holes!
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Joined: 02 May 2010
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PostPosted: Wed Jun 09, 2021 5:07 pm    Post subject: Re: Hit Miss Valve Stem Clearance Reply to specific post Reply with quote

Well, there is probably a spec somewhere but what I go by is if the valve reliably comes down on the seat and seals it's good enough for these old engines. Usually it's the exhaust valve that will give you problems first since it has the most travel. The intake never really opens, it just flutters enough (hence the weaker spring) to let a charge of fuel/air in. With the springs off close the valve and rock the stem side to side. You'll be fine with up to an eighth inch deflection at the tip of the stem. More than that and I would expect inconsistent sealing of the valve. Give the stems a shot of oil and push the valves open and close every time before you start it. Good Luck!
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