Piston - Valve clearence check

[Heinz911]

Hello All,

I'm reading and searching the forum already for some time but I now have a question that I don't find a post on.

I'm rebuilding my 2.2 T engine with original Mahle 2.2 S pistons. Part of the reason for doing this is that T pistons were not available and hopefully the domed piston will give me some extra compression. The case and heads were machined.

Now my question: I measured deck-height (0,7 mm) and piston to head clearance (1,0 mm) as explained in Wayne's book. Is this sufficient because of the more pronounced valve pockets in the S-piston in combination with the T-cam or do I need add a 0,5 mm cylinder base gasket?

Second question is it necessary to perform a valve to piston clearance check or can I trust that the deeper valve pockets in the S-piston in combination with the T-cam is a save set-up?

Thank you for your responses!

[Eagledriver]

I would expect it to have plenty of clearance, however, I'd check it during assembly. There are many threads on how to do this by turning the valve adjusting screws. Check the exhaust valve to piston clearance about 5 degrees BTDC and the intake valve about 5 degrees ATDC on the overlap. I like to see at least 1mm on the intake (one turn of the screw) and 1.5mm on the exhaust. Be careful to not jamb the valve into the piston with any force during these checks.

-Andy

[KTL]

Any time you're mixing and matching components, as well as having done significant machine work, you ought to check the clearance. Even a stock rebuild with "matching" components should do the check if some reconditioning was done on the parts.

Machining anywhere on the case (parting line and spigots) will decrease your piston to valve clearance. Milling your heads on the combustion chamber side will decrease the clearance too. So you should definitely check your clearances, just to be sure. It's a very simple procedure that you can do after you've set your valve timing.


Yep, agree 100% with Andy's recommendations. Turn the valve adjuster screw down until it contacts the valve (zero valve lash). Then turn the screw in one complete revolution for intake. Put a 19mm wrench (the idea is to use a short wrench, so you don't have a lot of mechanical leverage) and begin turning the crankshaft. Go slowly and watch the intake valve begin to open as you're turning the crank. You can also watch the piston come up the cylinder thru the spark plug hole.

You should be able to turn the crank quite easily because all spark plugs should be removed and you should only have at most four rocker arms installed. If you feel ANY resistance, stop, take note of where you are at on the crankshaft pulley. Then back up, and go forward again, proceeding with slow deliberate caution as you near that spot on the pulley. If you feel your wrench effort is substantially increased, you may be contacting the piston & valve

Do the same on the exhaust side with the screw turned in two complete revolutions from zero valve lash.

[boyt911sc]

Originally Posted by Eagledriver

I would expect it to have plenty of clearance, however, I'd check it during assembly. There are many threads on how to do this by turning the valve adjusting screws. Check the exhaust valve to piston clearance about 5 degrees BTDC and the intake valve about 5 degrees ATDC on the overlap. I like to see at least 1mm on the intake (one turn of the screw) and 1.5mm on the exhaust. Be careful to not jamb the valve into the piston with any force during these checks.

-Andy

Andy,

What's the theoretical minimum valve to piston clearances for 911 engines (example: 3.0 liter SC)? Assuming that the above 1 mm and 1.5 mm settings clear the piston, so you could safely use the new pistons or cams? Thanks.

Tony

[Eagledriver]

I don't know what the "normal" valve to piston clearance is for any of the engines. I know that a 2.7 RS replica engine I built had about 1mm on both the intake and exhuast. I machined the pistons for more clearance on the exhuast side. My SC motor has at least 1.5 intake and 2.0 exhuast (that's as far as I went on the checks). The only Porsche reference I've seen is for the early 911S engine. It lists .8mm as a minimum clearance for that engine.

-Andy