Does anyone have a rough hot to cold lash adjusting factor? The specs are for 0.028 clearance hot. For now I want to get a "reasonable" setting cold to make sure they are ok for the intial fire up (which is stil quite a way off).

Thanks - Greg

 

I've never measured a meaningful difference between hot and cold on my L-76, which makes sense because the thermal expansion coeffcient of steel and cast iron is about the same.

If you have aluminum heads and/or block, it's a different story. For example, the measured cold clearance on my Cosworth Vega engine is two thou less than "hot". The cold clearance setting on both inlet and exhaust is .014" (same lobe on both sides) and the tops of the clearance ramps are .016" above the base circle. Makes sense!

Also, "hot" is a relative term. With the engine fully warmed up and idling the exhaust valve is relatively cool. Run it hard for any length of time and the exhaust valve will rapidly heat up and close down the clearance a few thou, but that's why exhaust clearance ramps are typicaly taller than inlet clearance ramps - gives the exhaust valve stems more room to expand without hanging the valve open.

For example the Duntov cam inlet and exhaust lobes are identical except for the clearance ramps. The inlet side is .008" high and .012" on the exhaust side. At any point above the tops of clearance ramps the blueprint inlet lobe lift is .00400" less than the exhaust lobe.

Be careful how you index the engine when you set the valves. Many cams are still on the clearance ramps at TDC like the 30-30 cam. Set them at TDC, and they will be way too loose. If you set the inlets at 90 ATC and the exhausts at 90 BTC you will be okay. That way you can set two valves at TDC but they will not be the valves for that cylinder.

When you get back to #1 check the clearances on both #1 valves. If they are different than what you set at 90 ATC for the inlet and 90 BTC for the exhaust, then the cam is on the clearance ramps at TDC.

Duke

 

A hundred years ago, I read an article concerning the change in lash between hot and cold engines. There were ball park numbers for, all iron engines, aluminum headed iron blocked engines, and all aluminum engines. I forget the numbers, as I deemed them unimportant. The tip I DO recall, however, is to very carefully lash the engine, hot, per the specs. Then, come back when it is cold and measure the cold lash. From then on, you can lash it cold. One can play with the lash for tuning purposes, and therefore setting them somewhere close is acceptable. Assuming that you are asking for the initial fire up, just set them to the hot spec. They will probably be a little loose, which won't hurt a thing. As Duke stated, just be sure that the lifter is on the base circle of each lobe when you adjust it.

RACE ON!!!

 

Mine call for 18 and 16 thou hot, if I set them for 16 and 14 repsectively cold then they are pretty close when they heat up. That is 2 thou smaller gap for mine with aluminum heads and iron block. Crane says 6 thou in the blurb below but I would not go that big, somewhere between 6 and 2 thous will work for you if you have aluminumheads and iron block

"When installing a new cam, the engine will be cold but the lash specifications are for a hot engine. What are you to do? There is a correction factor that can be used to get close. We mentioned that the alloy of the engine parts can be affected by thermal expansion in different ways, therefore the amount of correction factor to the lash setting depends on whether the cylinder heads and block are made out of cast iron or aluminum.

You can take the "hot" setting given to you in the catalog or cam specification card and alter it by the following amount to get a "cold" lash setting.

With iron block and iron heads, add .002"
With iron block and aluminum heads, subtract .006".
With both aluminum block and heads, subtract .012".
Remember this correction adjustment is approximate and is only meant to get you close for the initial start up of the engine. After the engine is warmed up to its proper operating temperature range, you must go back and reset all the valves to the proper "hot" valve lash settings. "

 

Thanks. It's an iron block and aluminum heads combo. Guess I'll just set them to the hot spec for now. Just didn't want them to be too tight and hold a valve off the seat.

-Greg