Cam timing specs - HELP!!

[Rex T.]

I am trying to degree the cam in my new generation ZL-1 engine and am encountering some clearance issues (.035 piston to valve). The cam I am using is the GM 502-502 hyd roller cam, part # 12366543, using the stock GM hyd roller lifters. Pistons are TRW L-2300NF std bore. Head gasket is the GM unit for alum block and heads, .039 compressed thickness. Heads are new GM alum part # 12363390, oval port w/110 cc and 2.25 int valves. Crank is a new 6223 casting. Final displacement will be 427 cu in. EVERY piece in the engine is new.

Since GM does not provide valve timing specs for this cam, I was wondering if anyone has had experience with installing the cam in an engine. Total valve lift has been confirmed at .527 and .544 int and exh respectively.

I got these valve timing specs from a Speed Pro cam, part #KC1074R that was on EBAY:

@.006 SAE Tappet lift
IO BTDC 42
IC ABDC 74
EO BBDC 88
EC ATDC 40

@ .050" Tappet lift
IO BTDC 6
IC ABDC 38
EO BBDC 51
EC ATDC 3

The lift and duration are exactly the same as my GM cam, but when I use the valve timing specs for the SP cam, I have problems with piston to valve clearance.

Does anyone have any suggestions or contact info for GM Performance Parts help?

As always, thanks in advance for your help.

Rex

[Clem Z.]

Re: Cam timing specs - HELP!!

I am trying to degree the cam in my new generation ZL-1 engine and am encountering some clearance issues (.035 piston to valve). The cam I am using is the GM 502-502 hyd roller cam, part # 12366543, using the stock GM hyd roller lifters. Pistons are TRW L-2300NF std bore. Head gasket is the GM unit for alum block and heads, .039 compressed thickness. Heads are new GM alum part # 12363390, oval port w/110 cc and 2.25 int valves. Crank is a new 6223 casting. Final displacement will be 427 cu in. EVERY piece in the engine is new.

Since GM does not provide valve timing specs for this cam, I was wondering if anyone has had experience with installing the cam in an engine. Total valve lift has been confirmed at .527 and .544 int and exh respectively.

I got these valve timing specs from a Speed Pro cam, part #KC1074R that was on EBAY:

@.006 SAE Tappet lift
IO BTDC 42
IC ABDC 74
EO BBDC 88
EC ATDC 40

@ .050" Tappet lift
IO BTDC 6
IC ABDC 38
EO BBDC 51
EC ATDC 3

The lift and duration are exactly the same as my GM cam, but when I use the valve timing specs for the SP cam, I have problems with piston to valve clearance.

Does anyone have any suggestions or contact info for GM Performance Parts help?

As always, thanks in advance for your help.

Rex

the exhaust valve is closest to the piston at 10 degrees before TDC and the intake is closest at 10 after TDC. use a light valve spring,adjust the rocker arm to zero lash then set the engine at the above timing specs,put the dial indicator on the retainer and push down on the rocker arm till the valve hits the piston and take a reading on the dial indicator. you can check a couple of degrees on either side of 10 degrees to be sure. .035 is plenty on the intake and i would shoot for .100 on the exhaust