I am really going to show my ignorance here, but here goes anyway. I am having a problem with a new YS .63 and posted the question on their support line. Dave got back to me very quickly on the problem but his answer has me in a bit of a dither and he hasn't got back to clarify, so I am asking the question here.

He told me to adjust the valves to .003 mm and the only 2 feeler gauges I have are from an OS kit and they are 0.1 & 0.04. I always adjust my valves with the 0.04 guage and having walked through my last door at school some 40 years ago and not very good with decimals, I am assuming .003 is THINNER than 0.04, am I right ?

I have another question. The impression I get on everything I read is that the valve clearance in these engines INCREASES with a warm engine, this goes against everything I always thought was so in an automotive engine, is this true.

I'm guessing you mean .003 inches. The OS gauges are probably in mm. .1mm ~ .004 inches.

.003 is much thinner than .04.

I would set it somewhere between the .04 guage and the .1 gauge.

Your right, we are talking apples and oranges here. I went back to his answer and he has it written as .003". I did not see the" so I assumed he was talking mm. I guess I'll stay with the 0.04mm, YS calls for 0 - 0.1 in their specs.

Don:

A water cooled push rod operated overhead valve engine usually does have less running clearance on the exhaust than the cold adjustment, the exhaust valve is the hottest part, and expands the most. The intake is generally the same cold or hot.

Our model engines, having aluminum cylinder barrels, have more running clearance than the cold set clearance. Again, it is a matter of thermal expansion. The cylinder lengthens a lot more than the valves or push rods. This is why I always recommend their being set to 0.0005" clearance cold. On the Saito engines, at normal running temperature that leaves me with just about 0.002" running clearancce.

And while I would not object to 0.003mm cold clearance, I am sure, as BSC said, either you were told 0.003" or what was meant was 0.03mm. Just a hair tighter than the 0.04 stated minimum. If you use a feeler strip of 0.0015" you'll come out fine. That is 0.038mm, and you wont get it any closer.

Bill.

Thanks for the info guys. Bill, I could never quite understand about the hot & cold clearances in our model engines, but what you say makes sense. I spent too many years with my first love, car racing and adjusting the valves on solid lifter cams, I couldn't see the forest for the trees.

As far as the clearance goes on my YS .63 I was originally able to pass both the 0.1 & 0.04 guages TOGETHER under the rocker arm, so by setting it to 0.04 should make the difference and cure my problem. Again I couldn't see the forest for the trees, I was bound and determined that the valve lash should be OK after only 5 flights, another lesson learned. [&o][&o][&o]

Don:

Another car racer. My goodness. Then you know that after the rockers wear a little bit the feeler clearance is less than the real clearance. And the only accurate way to set valves is with a dial gauge. And that rocker wear is why I say to use the 0.0015" thickness, if you insist on using a feeler.

I have not used a feeler on a push rod engine for a very long time, except for instruction or demonstration. I have a P&G Valve Gapper that has served me admirably for more than 40 years now.

I was racing Triumph cars when I got it, and I got it at the recommendation of a fellow named Rob Walker. If you have followed GP racing, Rob had his own private GP racing outfit. What's more, I bought mine from Rob. More likely you will remember Jo Siffert. Jo was Rob's principal driver, when he was killed Rob gave up racing.

Anyway. On the model four strokes it is impossible to have the valves too tight and start the engine. At zero clearance they will still seal when cold, and you will have the greatest duration possible, and also the greatest power. I've never seen the tight adjustment hurt the idle, but I have picked up rpm on the top end by setting them tight. And there is less hammering in the valve gear with a tight adjustment, and therefore the least wear.

Bill.

Triumph cars eh ! Yep I had me one of those, a 1966 TR4A and a 1956 MGA, wire wheels and a continental kit on the back of all things and a 1957 'Vette and the list goes on & on, man I wish I had them all in my driveway now. All of my racing was done in a straight line or a circle, a 1/4 mile at a time. By 1973 the first "Kid" came along, I was broke and I had to give it all up. In 1984 that "Kid" fell into model airplanes and I followed, I've been having a blast ever since. As I say about my YS engines, R/C is a lot more bang for the buck.

Boy - do the P&G valve gapper comments bring back memories of the good ole days of racing! Being in the auto repair business back then, I used the P&G almost daily. (I was one of those guys who always removed the distributer to install points and verify operation on a distributer machine too.)

Of course your explanation of rocker / valve gap changes with thermal expansion are right on. I would add that in order to minimize valve train wear (especially cam lobe / follower) in our model engines, the gap should be maintained at the minimum. There is precious little, if any, lash ramp ground into the cams. I've found that the YS engines require a tappet adjustment after the first 5 tanks or so to account for breakin wear, then every 50 flights or so.

Earl