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Valve lash when the engine is running

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Is there documentation that reveals what the running lash is vs. the cold set lash on a CRF450R or similar engine?

 

I have read on formula 1 engines the lash is set to zero when the engine is hot and when cooled the valves hang open and will not start. This process seems to be done before each race (setting and re-setting the lash). I would think the same situation applies to the dirt bike in terms of thermal expansion of the valves.

 

I always thought the lash decreased just as an observation due to the valve train appearing more quiet when the motor is hot.

 

The typical lash for the exhaust is .010"-.012" EXH and .005"-.007" INT.

 

What I wanted was a lash that will yield the best power without burning a valve and a valve train that would be the best on the valves hitting the seats.

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The F1 thing sounds a bit like BS, but I don't know.  There are a couple of them that use hydraulic tappets that are manually shimmed to a very small pre-load so that they always run at zero clearance and yet cannot hold a valve open more than a very few thousandths if valve float should occur (and in an F1 engine, it generally never does).  I do know that on Yamaha YZF heads the valve length increases less with normal heat than the increase in the distance between the camshafts and the valve seats that is due to the expansion of the aluminum head.  That's one of the processes by which a valve can recede to "past zero" clearances when checked cold. 

 

The thing to remember is that the valve isn't effectively open until it reaches at least .015" lift, and if you degree your cams, I think that you would find very little difference in terms of duration between factory clearances and anything tighter than that.  IMO, it's not worth risking a valve over when you could much more reliably just upgrade the cam itself.

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My thoughts were the valve head expanding and seat. This would cause the stem to be pulled away from the camshaft. Ideally, I would like to run as tight as possible to slow the valve down to it's seat. It was reported that many with radical cams use .004 and .009 clearance to help the valve train live. If the lash is too great the valve can cup form bouncing the seat. Not sure any truth lies in this. Just looking for another perspective.

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op's crf gets looser when hot, I've pulled the valve cover off and measured to prove it to myself.  You need to maintain some clearance of the rocker roller so you can't tighten the exh too much, it also might effect the decomp timing but I haven't seen that being an issue.  Intake side I've run em as low as .003 with no issue.

 

the ramps on the cams are surprising pretty slow so some tuners have tried going the other way, looser, trying to get more time on the seat and quicker opening,  I found it to be quite noisy.

Edited by highmarker

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op's crf gets looser when hot, I've pulled the valve cover off and measured to prove it to myself. 

 

 

 

Same as the YZF, then.  That's what I would have assumed.

 

the ramps on the cams are surprising pretty slow so some tuners have tried going the other way, looser, trying to get more time on the seat and quicker opening,  I found it to be quite noisy.

 

If you were looking to increase power by tinkering with clearances, that would be the approach to take.  Try it with a degree wheel both ways and you'll see that looser will produce more duration at .020"+ lift than tighter does, just because of the ramps.  But the ramps are important in addressing the valve bounce issue mentioned by the OP, too.

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 straying off topic, sorry

stock lash has the valve closing at 1.2 fps calculated at 12,000rpm , which might be considered slow. 

 

 what it looks like at .005 

.005 lash.jpg

 

 

and at .008,  note change in location of green closing line

.008 lash.jpg

 

 

 

 

Edited by highmarker

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