tappet clearances

Phil · Posted: Wed Apr 10, 2002 2:36 am Post subject: tappet clearances

Can you get a bit more grunt by stuffing around with the tappet clearances?? I remember reading something in Adam's gertrude stuff in the workshop. It makes sense to me that you could push the valves open a bit more but wouldn't that make the valves not close all the way? What clearances are most of you peoples running?

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

Your thinking is right. You won't get much more by tightening the clearances and you would then have the valve not sealing properly. Remember at 6000rpm, these things are opening and closing 50 times a second. They need a little bit of clearance for flexing and overlap. Only a camshaft change or rocker geometry change would make a significant difference.

Yonnee.

tdwepn · rice boy Joined: 10 Apr 2002 Posts: 41

gday
if i have a 30/70 cam in my motor does it change the tappet clearances that i sould have as apozed to the standerd tappet clearances?

if so what should i set my tappet clearances to ? (coz im gonna do it tommorrow arvo, any advice would be appreciated.

thanks fellaz

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

No, the clearances should be the same. The camshaft doesn't change anything when the valves are shut, only the height and length of time they're open.
Inlet;.........0.15mm cold, 0.20mm hot.
Exhaust;...0.25mm cold, 0.30mm hot.

Cheers,
Yonnee.

Phil · Posted: Wed Apr 10, 2002 12:46 pm Post subject: Re: tappet clearances

Thanks Yonnee, I like it when I'm right! Razz

Momo · backyard mechanic Joined: 08 Feb 2002 Posts: 561

Hey? I asked this very same question a while back and was basically told that every cam was different and that you should set your valve clearances according to your individual cam. I put my gem in to be dyno'd at the same time as having the head re-tensioned and they set the clearances to .08mm exhaust and .10mm inlet - now I'm really confused.....

GasGem · backyard mechanic Joined: 05 Nov 2001 Posts: 755

I must agree I have to run .08mm and .10, other wise they make a racket, and I have found this with a Camtech cam and a crow cam,

Cheers
Eggy

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

OK. I'll expand a little and hopefully make some sense.
OEM factory specs for everything are a comprimise. Tolerances are not as tight and everything is manufactured in bulk to save on costs. Aftermarket, on the other hand, is supply and demand driven, particularly performance parts. These are manufactured to much more stringent specifications because, as the old saying goes, "you get what you pay for". Imagine the uproar if people paid top dollar for parts that were "near enough". In short, you probably could reduce your rocker clearances to 0.08mm and 0.10mm on an OEM cam but could you guarentee that the cam specs were that tight.

Yonnee.

tdwepn · rice boy Joined: 10 Apr 2002 Posts: 41

hey eggy i got a stage 3 crow cam im currently running the standerd clearences , if i run .08mm .10 clearences what difference would it make? if any? any more hp?

cheers m8

Twisted_torque · tinkerer Joined: 26 Feb 2002 Posts: 240

I have a stage 4 crom cam... I got it out of a car at the wreckers for jack all cause they though it was standard. Neway is this cam legal on the street and on a 2L with shaved 1.6L big port head is it gonna run with no hassles. Crow cams told me that the tappet clearances are different for it but I lost the sheet I wrote it on if anyone has the catalogue or knows can they help me out too ...

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

See what I can do.

Yonnee.

Ben Wight · backyard mechanic Joined: 04 Oct 2001 Posts: 946

The tappet clearance is purely to counter the effect of valve expansion with repspect to temperature rise. Thus, particularly in a high hp engine, where exhaust valves in particular are going to get hotter than usual, OEM specs on valve tapped clearances should be observed.

If the clearances are made too small, the valve might be kept open when really hot, and thus the only thing you will have achieved is a miserable drop in performance as all your cylinder pressure escapes out into the exhaust.

You also have to think about how much extra lift you are talking about, which is basically fuck all and will have about a 0.01% performance benefit, untill the valves get hot that is. If you really want to get more gas in the chambers, get a bigger cam. You have got the rocker cover off to adjust the valves anyway, why not go a step further and install a cam, its only a 1/2 hour job.

PS, I run 6 though inlet and 10 thou exhaust clearance on warm valves. Thats about 0.15 and 0.25mm respectively.

Ben Wight

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

The long term effect of having the valves bordering on being kept open is you'll burn them out in double quick time.

Yonnee.