Valve clearance changes as engine heats up.
Valve clearance changes as engine heats up.
(OP)
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
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





RE: Valve clearance changes as engine heats up.
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.
RE: Valve clearance changes as engine heats up.
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
RE: Valve clearance changes as engine heats up.
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!
RE: Valve clearance changes as engine heats up.
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.
RE: Valve clearance changes as engine heats up.
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
RE: Valve clearance changes as engine heats up.
RE: Valve clearance changes as engine heats up.
Rod
RE: Valve clearance changes as engine heats up.
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!
RE: Valve clearance changes as engine heats up.
RE: Valve clearance changes as engine heats up.
Air cooled engines seem to be more affected than liquid cooled engines due to a better thermal control with liquid coolant.
Franz
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RE: Valve clearance changes as engine heats up.
RE: Valve clearance changes as engine heats up.
alum block 10.2 inch deck height
alum heads
valve lash change (push rod motor).010-.012 cold=.022-.024 hot.
RE: Valve clearance changes as engine heats up.
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.
RE: Valve clearance changes as engine heats up.
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