I just got my E31 head back from the machine shop. I had them install new bronze valve guides, new hardened valve seats, stock L24 intake swirl valves and L28 (larger) exhaust swirl valves with the corresponding larger hardened seats. My question is, the stem height on the exhaust valves are higher than the intakes. I'm installing an Isky Stage II cam, and to get proper rocker geometry, I need a .190 lash pad to center the intakes on the new rockers and a .160 lash pad to do the same on the exhaust. Is it OK to have the two valve stem heights different since I can get geometry right, or do I need to go back to the machine shop and have them adjust the valve stem height so that they are the same? If I need to go back to the machine shop, how do they make the adjustment? Thanks Guy

Guy,

I got a head back from a shop once where every valve in the head was sunk to a different depth. It took many hours to get the wipe pattern the same on all the rockers but I did get there. I then ran the engine and it worked perfectly in all aspects.

Your job will still take some time as I would mock up each valve to see if the inlets and exhaust valves are set to the same depth. You know this...assume nothing and then there are no nasty suprises

Because you're centering the wipe patterns on the rockers with different lash pads, you're ensuring a correct rocker ratio for each lobe/valve in your head. All valve lifts and durations will be the same because of your efforts here.

Good luck mate...all will be well with your attention to detail (not all reconditioners understand the 'geometry' needed for the L head... unfortunately!)

(PS Stock cams or those with small valve lifts are ok but be more cautious with aftermarket grinds. The lifts are higher and the durations longer which could bring the valves closer to the piston crowns. Any contact with the piston usually occurs with the exhaust valve as the piston chases the exhaust valve at the end of the exhaust stroke.)

Are your bores notched for the larger exhaust valves your going to use? I know L26 blocks are, but I don't know about L24 blocks. Someone else can chime in if they know this stuff better than I do.

Cheers :classic:

Oz....Thanks for your response. Yes I'm checking for proper geometry on each rocker. We installed new hardened seats which as I understand it can be somewhat different in thickness than stock. The intake valves all center the patch with a .190 pad while the exhaust require a .160, so at least there is only 2 sizes required. Yes , since we installed a larger 280 exhaust valve, I notched the block for safety even thogh I'm bored .040"over. One more issue is contact with the valve seal (the Isky valve retainer contacts the seal). Can I have the machinist grind more on the seat to increase clearance or is that a no no. I'm going to install a lower seal (Datsun "A" seal or Felpro Ford seal)to increase clerance.I can't imagine how someone with a .600 lift cam can make the seal clearance needed . Thanks Guy

Hey man, seems you have a good handle on what you're doing. I thought about your option of sinking the exhaust valves deeper into the head to clear the valve stem seals from contacting the retainers. That certainly is an option. Using the "A" series seals is also a good option. Now it boils to costs (sinking the valves may require spring seat shims to get the correct valve seat and full lift spring pressures) and whether or not you ever intend on using a different cam in the future that has a higher lift than the one you currently working with.

It'll be good to see some you tube action (or a dyno sheet) of your work when your done....I'll make a point to turn up the volume on the speakers

All the best Guy!

Cheers.

Cheers to you too OZ.....I'll keep you informed. Thanks for your help. Guy