Valve clearance changes as engine heats up. 1

[DesignerMike]

I am working on a motorcycle engine and got into a discussion about how the valve clearances will change as the engine gets hot. It's on an inline 4 cylinder with 5 valve per hole. (small valves)

His claim was that the valve stem gets hot (expands) and reduces the clearance with the camshaft.

I disagreed since the head and other parts are also expanding (making it thicker).
Actually I feel the most important part is that the overhead camshafts spin in an aluminum "carrier" which grows much faster than the valves. The camshafts are probably about 1" of aluminum above the valve stem guides

The spec is for .003"-.005" clearance when "warm"
At the end of the day the thermal changes are probably pretty insignificant but I was interested in getting some other feedback.

Thanks,
Mike

 

[ivymike]

Thermal changes in valve lash are almost certainly not insignificant. They are critical on most engines I've seen. note: valve lash is not the same as valve-to-piston clearance (just in case that's what you meant by "valve clearance")

Whether the clearance grows or shrinks will depend on the temperatures and materials of the various components involved. The valves get very hot, but usually have a lower thermal expansion coefficient than the head (Al?)
valve, say nimonic 80a, 12.7 µm/m-°C
head, aluminum alloys, 23-26 µm/m-°C

I don't know what a reasonable temperature profile looks like for a motorcycle head or valve.

I have at hand the data for several medium and high speed diesels, but suspect it may look considerably different than that for your bike. For the diesels, whether cam-in-block or overhead cam, lash shrinks in the hot condition (all materials are around the 14 µm/m-°C range). This suggests to me that valve elongation is dominant in these engines.

 

[evelrod]

Not having access to the information that Isaac has, I can only give some anecdotal cases...
Lotus dohc...lash increases slightly (.001" to .002") from cold setting to hot (warm) settings as checked by feeler gauge.
Norton motorcycle (1948 ES 2) Lash increases a very small amt from the cold "nil" setting to hot operating temps.
'63 Mini Cooper...lash decreases slightly from cold to hot.

Some engines are set hot (warm) and some cold. The above is referenced for "cold" to "hot" for continuity as it relates to the OP.

I agree with Isaac's reasoning that the valve elongation may be a dominate factor. Perhaps in the case of my DOHC engine experience the factor appears less dominate because of the Al head and Fe block?

Rod

 

[DesignerMike]

Thanks for the responses.
If it helps, the specific engine is a Yamaha XJ700X
700 cc DOHC, 5 valve genesis head

It utilizes the same head and valve train design as the fazer FZ family

It is a major PIA to adjust the valve lash since the engine uses a "shim under bucket" design and you need to pull the cams and buckets to swap shims after you have carefully measured clearances. There is no way to know what shims you have until you pull the buckets so you almost need to do the job twice unless you have a huge supply of different dimension shims in the parts box....and of course the darn little things are about $10 each!

 

[BrianPetersen]

I know exactly the job you are talking about although I haven't worked on that particular engine. The FZR and later R1 engines are very similar.

If it is like the other Yamaha engines, the intake clearance spec will be 0.10 - 0.20 mm and the exhaust clearance spec will be 0.20 - 0.30 mm. This is because the exhaust valves run VERY hot. The cylinder block is of course aluminum, but its temperature is equalized out by the cooling jackets and the average temperature won't be much different from the cooling system, but the portion of the exhaust valve between the valve guide and the valve itself will be really, really hot (sorry, don't have numbers, it'll vary with engine load though). The intake valves are exposed to combustion conditions on one side but are cooled by the intake charge.

The extreme temperature of the exhaust valve stem will make that part of the stem expand more than the aluminum head for sure. The exhaust valve clearance will run tighter when the engine is hot. The intake valve clearance probably won't change much.

 

[evelrod]

Even better, Mike...I had a '77 KZ 650 Sport for several years and some 40k miles that was quite similar to you XJ...

I have many years of practice with the DOHC setups and most have the lash pad under the tappet as does your bike. On the KZ the lash was something like .002"-.004" cold, if memory serves for the stock cams. Hot settings, really just warm as by the time you get everything off, it's just warm, were still in the same range. Even though ALL cold settings were "spot on" the hot settings varied several thou, one to another.

Each camshaft design is different as to "lash". I designed a set of cams for my Lotus back in the mid 80's (with a lot of help, I might add) and set my lash at .012"-.014" from the get go. I suppose I could have used something different, but chose a larger gap as to forestall any possibility of closing the gap to the point of failure, particularly on the exhaust valve. Ex valves tend to run a bit hotter than In as a general rule.

In dyno tests with a set of Cosworth cams, using various lash settings, HP figures did NOT vary a significant amt. However, mid range TQ did wander a bit.

I stick with the cam grinders specs for the most part. Not slavishly, but within reason...unless testing warrants.

Rod

 

[TrackRat]

So yes the valve lash can change considerably depending on the head/valve train design and materials used. When in doubt the only true means to know what is actually happening dynamically through the entire temp range is to measure the lash at different temps.

 

[evelrod]

Yep, TrackRat...I'll give ya a star for stating the obvious in two sentences when I did not seem to do it if two paragraphs plus! ;-0

Rod

 

[DesignerMike]

Ah, I wish it wa so easy....you have to take part of the cooling system apart to get at the camshafts. I imagine I could temporarily rig something up but I'm not that curious.

I was mostly after some good discussion about what might be happening (which I got)

I'll adjust them as recommended in the manual knowing that the yamaha engineers knew exactly what they were doing when the wrote the book

Thanks for all the good responses!

 

[TrackRat]

Oh you're a poor sport! You don't know how much fun it is pulling apart blazing hot engines to measure components and clearances. Have you no life? <LOL>

 

[franzh]

One early hot rod trick was to use aluminum pushrods from (I think) an aircooled VW. The big problem was the aluminum would expand more aggressively than the cast iron block and heads and remove any valve lash present causing burned valves.

Air cooled engines seem to be more affected than liquid cooled engines due to a better thermal control with liquid coolant.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TrackRat]

There are all kinds of "tricks". Formula V engines typically run zero hot lash to get as much cam duration as possible. They warm the engines up before the race and then set the lash to zero. When the engine gets cold the valves are open and it won't start until you back them off so they are closed.

 

[racecrafter]

another example
alum block 10.2 inch deck height
alum heads
valve lash change (push rod motor).010-.012 cold=.022-.024 hot.

 

[BrianPetersen]

That stands to reason, because the aluminum block and heads aren't too different in temperature from the pushrods, and would be expected to expand faster.

The original poster's engine has direct-acting overhead camshafts. The camshaft pushes on a bucket with a shim underneath, and that pushes directly on the valve stem. No pushrods no rocker arms no nonsense no nothing.

 

[tbuelna]

Aluminum has twice the CTE rate of most ferrous valve materials. The average operating temperature of the N/A exhaust valve would likely be about 3 times higher than the aluminum head material, and the intake valve would be about 1.5 times higher. The startup conditions would be different, with the exhaust valve heating up faster than the head. Even with the short valves in your Yamaha, the CTE mismatch can easily be .006 to .008 inch at operating temps.

You never want a condition where there is negative valve lash, which would keep the valve off of its seat. But having a minimum valve lash during operation is also very important, especially with high speed valve trains. Excessive valve lash will result in high impact loads as follower and cam lobe come into contact during the initial valve opening period as the lash clearance is taken up. A difference of .010 or .020 inch can increase the contact forces by several factors. So this lash tolerance value and CTE mismatch between the valves and head are not insignificant.

Good luck.
Terry