Pushrod engine valve lash

[maqqx]

Until today I was sure the pushrod-type OHV engines tend to increase valve lash when they get hot.

But today it turned out my 156fmi pushrod engine probably does exactly the opposite.
I went to pick up my bike from the garage and the mechanic told me he had to increase the clearence on the intake up to 0,13 mm because the valve was getting too tight (there was no lash at all) after the engine had heated up.

What dou you think?

Now my bike runs just fine.

[franque]

Valve lash always decreases when hot, due to greater thermal expansion in the aluminum parts than the steel/iron parts. That's pretty normal. That valve clearance sounds a bit loose to me, it should be closer to .08mm, but if it is running okay and not making too much noise, you should be good!

[maqqx]

I thought this was the case with OHC engines with a timing chain, but not with the pushrod engines. I'm even pretty sure I've read about it in an old CG125 manual. Well, must've been wrong.

[franque]

Every engine has lash specifications because of thermal expansion. The idea is to get the clearance as close to zero as possible when the engine is warmed up. You were operating on incorrect info.

[maqqx]

There's a good chance you are right about the incorrect info. But where to get the correct data from?
My source of information was the official service manual I got with the bike.
I had also asked a friend to look it up for me in Haynes Chinese Motorcycle manual. Values differ slightly but it's always close to 0.08 mm. Never a whopping 0.13 mm.

[mtiberio]

any valve lash makes your motor (or more specifically your cam) less efficient. Any lash has to first be squeezed out by the cam lift before said lift starts opening your valve. Cams are all about lift of the valve. When I was racing, if someone has clearance issues with their valves from a high lift cam (hitting piston, hitting each other), they might run clearances up to .5mm (.020") or more. As an engineer, you want to design the valve train and suggested clearances so that you have close to no clearance while hot.

[maqqx]

I'm not an engineer but if I understand you correctly it is fine to run a bigger clearence as long as it helps?

[mtiberio]

you could get into all kind of arguments about cams and lifters slapping. I'd run what the factory says unless I had a real reason not to.

I assume your bike was not running fine before you went to the mechanic? Measure what he did when the bike is cold and experiment on your own. You are not likely to damage these stone bridges. Just don't ride forever with the valves tight.

[Megadan]

I thought this was the case with OHC engines with a timing chain, but not with the pushrod engines. I'm even pretty sure I've read about it in an old CG125 manual. Well, must've been wrong.

pushrod engines have multiple things that create a more dynamic system.

The valves and valve springs all expand from the heat, decreasing the valve clearance at the point of contact. The cam itself increases in size as it gets hot, as do the followers/lifters decreasing the total clearance avalable.

Those last points apply to all engines.

Specific to pushrod engines though...

The cylinder and cylinder head expand away from the cam in the lower engine case. This actually increases the gap between the valve and lash adjuster. The pushrods themselves also expand as they get hot.

In most engines, the pushrods are aluminum, which being of the same material as the cylinder and head, will expand at close to the same rate, thus not overly altering lash too much.

Since the followers expand away from the camshaft on the design of this engine, themselves being secured to the cylinder, they actually expand away from the camshaft more than the camshaft grows in size. The pushrods, even though they are aluminum, still don't expand at the same rate as the cylinder because they are cooler and kept at a more constant temperature. These two factors actually cause these engines to increase their lash spec a little as they get hot. That is one reason the lash spec on these motors is actually quite tight, even tighter than the OHC variants. .05mm minimum spec vs. .08mm minimum for most of the OHC engines.

That is why many people complain about these engines being noisy once they get hot.

Change the pushrod material like I did to a high carbon steel, which expands a lot less than aluminum, and the lash gap increases even more as the engine gets hot. My valves are set to a literal zero lash when cold, but when hot my motor is quite clattery.

[maqqx]

Megadan, that's what I was taught too.
So I don't understand how's that possible my bike had bigger valve lash when it was cold. Beacause that's what the mechanic said. The intake was set to 0.08mm by me and adjusted to 0.13mm by the mechanic.
And the fact is it is stalling no more.

[Megadan]

I ran my valve lash at .06mm on the intake before I installed the chromoly pushrods. Never had one issue.

I would say there is another problem at work. too loose or tight valve lash can mask fuel tuning issues, or amplify them.

[maqqx]

Do you think the root cause of my problems could be deterioration of valve ports?

[herbie]

Until today I was sure the pushrod-type OHV engines tend to increase valve lash when they get hot.

But today it turned out my 156fmi pushrod engine probably does exactly the opposite.
I went to pick up my bike from the garage and the mechanic told me he had to increase the clearence on the intake up to 0,13 mm because the valve was getting too tight (there was no lash at all) after the engine had heated up.

What dou you think?

Now my bike runs just fine.


On my bike I would set my intake to .11mm and exhaust to .13mm, later I changed my exhaust to .11mm, and it would strong that way. I was always with the school of thought that a tappy valve is a happy valve. I would rather have it on the loose side than too tight. I saw a video years ago from an engine builder that turned to motorcycle repair about valve clearance and it's effect on engine and carb performance, I looked but couldn't find it.