Hey gang, this is my first post in this forum, (long time member of the other Zcar site). My car is a '73 240Z, mostly original, overhauled suspension, pertronix ignition, 3-screw SU's, nothing exotic.

Anyway, doesn't seem I can do any major project without one good "learning experience" involved. This winter I did the timing chain set, water pump, intake gasket, clean polish, paint etc. Was considering the on-head valve seal job, using compressed air to hold up the valve. I was playing around with adapters to get the air into the cylinder, and it turned over, and and stopped with a thunk. Unfortunately, I had the timing chain off, so that thunk was the #3 piston hitting the intake valve. I backed it up, got it timed, started it up, and it ran like crap. Long story short, #3 intake is stuck open enough so I get zero compression, backfiring, low vacuum, etc. So I know the head needs to come off, no big deal, good excuse to get the valve seals.

My question is far do I go with the valve job? The engine has about 120K, great compression (170 psi +/- 5), and oil pressure. I don't want to pay for a complete head job if I don't need it, but I am willing to pay for what makes sense at this point in the engine life.

Job could be anything from just replace the bent valve and seals, to popping them all and grinding, replacing the guides, new hard seats etc. With my decent compression numbers, I don't see the need for total rebuild any time soon with the low miles I put on it, and anyway would like to save that money for an L28 upgrade, or stroke the L24.

It seems these cams and rockers are sensitive to change, and I really don't want to get into a complete top end rebuild I don't need it.

Moral of the story- don't turn over the engine without the timing chain, even by hand!

Thanks gang

Eric

Sorry to hear your misfortune.

It will probably not be an option that would suit you but an interesting incident I witnessed one day at a race meeting at Winton raceway here in Australia. A friend bent a valve in his datsun sunny (I can't remember how) and to avoid missing the rest of the day they whipped off the roker cover, and rocker. Put a socket on the end of the valve and gave it a good number of belts with a hammer to comperss the spring all the way down. (making sure the piston in that cylinder way not at TDC.

When the valve sprung back and hit the valve seat it straightened the valve enough for him to complete the day. I didn't know him too well at the time (I do now) so I'm not sure if it fixed it enough or if he had to do further repairs when they got home.

Do the entire valve job......If you must pull the head anyway it makes sense. More problems have been caused by grounding the starter.....That's why you should always disconnect the battery when doing engine work.

Yeah, you need to take the head off.

It's your baby man, how could you skimp on her now?

Do it once and do it right!

Cheers.

Edited April 29, 200915 yr by ozconnection

Ya, I hear you both- but what does a complete job mean on an OK engine- new seats, guides, grind whats there and but it back together? Especially the seats, I understand the '73 had bronze seats, should I plan on them too or just a grind? Also, If I grind the seats, will I need to get new different thickness lash pads so i don't wear out the cam and rockers?

Grrr.... thanks

Ok, more info. Popped the head, and indeed the valve was bent about 1/2 open. Very small nick in the piston, otherwise seems OK. took rockers and cam out, measured valve wiggle at the top, slightly open, with dial micrometer, parallel to rocker travel. Intakes seem pretty tight, at around 0.001"-0.002" deflection, seats look good. Exhaust is another story, with about 0.004" deflection, a noticable "wiggle" at the valve head and some pitting. Need to get a more accurate mic to measure valves, but assuming they are OK- here's hat I'm planning.

New exhaust guides, new seals, grind all seats and valves. Should I have them grind the tips of the valves to restore them to the original installed height, or just bite the bullet and adjust wipe pattern with new lash pads? Will a standard regrind change the installed valve hight enough to matter? Anything else I should do while in there?

Does this sound like an OK plan?

Cylinder bores look great for 120K, no scoring and can still see the factory honing marks. Warm compression was 170-180 PSI in the good cylinders.

Edited May 5, 200915 yr by Mn_Z_Man
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