Valve clearances on a dirt bike??

My clearances right now are 0.03 both exhaust and inlet on a 70 size dirt bike but on other models of bike the company make which are bigger cross country style bikes with the same engine the clearances are like 0.05 in&exhaust which gives it a slower rev but better top end feel etc... I've seen road 125s with clearances at 0.09 inlet and 0.10 exhaust which can do like 90mph so if I wanted to put some road tyres on my dirt bike and register it for street use would it be worth adjusting the clearances??? I'm talking for long distance??? Is there any rules that apply or should I always use oem recommendation

Robert J

Valve clearances are not for tuning. They are to ensure that while the engine is warming up and once the engine is up to maximum temperature, the expansion of the various parts does not cause the valves to be held open and leak/burn. The settings will vary from engine to engine due to physical sizes of parts, materials used and expected working temperatures. Always use the manufacturers setting.


i would just go with oem, you just need enough clearance or your going to have problems hot starting etc ... if you have a response issue i would look more at jetting and make sure you have the proper plug in it and adjust the heat range as necessary ...

Tim D

"I've seen road 125s with clearances at 0.09 inlet and 0.10 exhaust which can do like 90mph" No you haven't. Changing the valve clearance is not a tuning mod (unless you have changed your cam).


Stick to the OEM setting. You want more power, put a racing cam and adjust the clearance to the racing cam spec.

Candid Chris

Most small dirt-bikes are internally geared to go slower so no matter how you tune or change the external gears it still will not be really that great for street use.


Depends on ACTUAL dirt bike. Is this a Chinese clone of OHV Honda or OHC model? OHC tends to have smaller lash settings than OHV with extra length of pushrods Tappets, rockers. OEM specs are best settings for most riding, Maker paid engineers some $$ to get tuning specs right. Valve lash setting are to be checked at regular intervals usually, especially if no Hydraulic tappets like Harley had stating 1950s. You want loose fit for cold start, a little tapping/rattle normal then. hot engine should be quiet, maybe just faintest of 'tick'. Quiet cold may mean not enough lash- and if hot some backfire or pop on exhaust means check settings- burned valve possible yet. Heat transfer from valve facing to valve seat occurs when seated- not when spaced between seat and valve face- that means edge of valve will glow, burn some and maybe 'spot weld' to seat that would erode some also. 1960s, early 1970s motors that were made for leaded fuels were getting badly worn using the unleaded fuel that came on market when catylitic convertors became standard on cars. 1980s when leaded fuels became hard to find meant a lot of worn valve seat, worn guides- BMW /5 and 6 ,R/2s were advised to do adjustement 2000 miles instead of 4000 miles, lash setting .004/.006 to be increased to .006/008, because the valve seat erosion, Harley Panhead, shovel heads were advised to check lash- hydrualic lifters, pushrods were reaching end of play and lot of seat erosion, bad valves. Dirt bikes have rough service - idling at low speed with burst of high RPM or just high RPM when racing, playing in dirt. if maker has .005 intake and exhaust for street engine same 70ccs, you notice no tick when hot or some exhaust popping, then check las setting and maybe reset to .004-- split the difference of maker between dirt and street engine. Shop manual may give range of lash- as in .003 + .001