Valve guide clearance R12?

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pkrpkr
Posts: 10
Joined: Sat Jun 25, 2016 10:58 am

Valve guide clearance R12?

Post by pkrpkr »

The cylinders of my R12 had the holes for the valve stems worn so one of our specialists put in brass inserts. After the first couple of minutes riding the cylinders got hot (200+ Celcius), an exhaust valve got stuck and the valve guide had a press fit which was a bit too loose so it had moved.
The specialist redid it and I checked the clearance between the valve stem and valve guide to make sure it was OK. It was 0.07mm (diameter) so that looked good to me.
Apparently that wasn't enough because within 2km after assembly again the exhaust valve got stuck.
Does any of the veterans have a recommended clearance for these old sidevalves? I think I'll go for more than 0.07mm but what is reasonable before it gets too much?
Thanks for any advice,
Peter

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C.K
Posts: 148
Joined: Wed Jan 08, 2014 11:58 am

Re: Valve guide clearance R12?

Post by C.K »

install the original it will last forever
clearance of 0.1 up 0.8 its ok.

Hans
Posts: 23
Joined: Wed May 10, 2006 5:12 am

Re: Valve guide clearance R12?

Post by Hans »

in 0.15mm, out 0.20mm. The manual says for both 0.05 mm less which is too tight.

pkrpkr
Posts: 10
Joined: Sat Jun 25, 2016 10:58 am

Re: Valve guide clearance R12?

Post by pkrpkr »

Thanks for the input. On further measurement I discovered that the valve guide hole was not exactly straight.
So even with the proper inner diameter of the valve guide at all spots and it moving freely at room temperature, the exhaust valve stem could have gotten too close to the valve guide sidewall once the cylinder reached normal working temperature. Causing it to rub against the sidewall causing more friction heat than normal.
Just my theory. We'll see after reaming to a straighter hole with a piloted reamer.

Ed.: reamed inlet to 0.07mm and exhaust to 0.1mm. Seems to be OK now.

spider rust
Posts: 135
Joined: Mon Apr 07, 2014 2:45 am

Re: Valve guide clearance R12?

Post by spider rust »

The cylinder exhaust valve ports can be blue printed, opened up to allow better smoother and more air flow over the valve seats and out of the exhaust ports, reducing the heat build up, increasing performance, and with the bonus of modern fuels is much better. Remember that engine was designed to run hot and to burn up low grade fuels used
during WW2.

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