4 replies to this topic

[MiniHead95]

I have 1.5 roller rockers and wondering what I should set the tappet clearance to, I've heard 15 thou for 1.5 and standard, 12 thou.

Also I've heard the cam and engine can sometimes make a difference so here is the spec.

1310, Kent 276 cam, mini spares 1.5 roller rockers, Ported and flowed head, big valve metro head. Think that's everything that could contribute to it being a different thou.

Cheers for any answers :)

[racingbob]

I have 1.5 roller rockers and wondering what I should set the tappet clearance to, I've heard 15 thou for 1.5 and standard, 12 thou.

Also I've heard the cam and engine can sometimes make a difference so here is the spec.

1310, Kent 276 cam, mini spares 1.5 roller rockers, Ported and flowed head, big valve metro head. Think that's everything that could contribute to it being a different thou.

Cheers for any answers :)

I used to run 15 thou on my race mini had no problems reving to 8000

[Cooperman]

0.015" is fine, but 0.012" would also be OK.

The reason for the gap is to ensure that as the valve expands there will still be a gap so that the valve closes fully.

So a valve will expand the same amount for a given temp. increase. The only difference a higher ratio can make is in respect of the expansion of the push rods, which is minimal compared to the exhaust valve. The inlet valve expands very little as it runs quite cool.

You could set 0.012" and it would still be fine.

[Ethel]

The gap's across the "see-saw" of the rocker as you need to be up against cam and the valve stem to take the slack out. Hence it's proportional to the rocker ratio and you can adjust accordingly - if 1.5's offer about 20% more lever you want 20% more clearance.

[Spider]

The gap in part is to allow for Valve Stem growth from cold to hot but mainly to take up Valve Train lash on the Clearance Ramp of the Cam Lobe, before the follower is on the Opening Flank, Similar on the closing side.

 

Put another way, it also stops the valve train from getting hammered to death in very short time.

 

Normally, as Ethal mentioned, you'd increase the valve tappet Clearance pro-rota in accordance with the change in rocker ratio, then the clearances from the Push Rod down will be maintained.