N42 head / valve job
 

Hey guys, sorry I've been MIA for a while. had some non-Z stuff going on that I had to tend to for a few months.

Earlier this month I had started my 76 280z, which is in the process of getting some body work done (doors off, fenders off), addressing some of the rust/cancer issues.

Anyway, after it had run for about 20 mins I peeked into it and steam was coming from the passenger side of the block where it meets the head, sure enough, the head gasket was leaking. I was overdue for it anyway.

So this weekend I pulled the head, thankful for all the anti-sieze I used on the exhaust and intake manifolds. One of the head bolts snapped off in the block, but I can handle that issue.

I found a machine shop that will clean the head for $15 USD, and they will true the gasket surface for $60 more... to me that's going to be money well spent.

I have two questions... first, are there any valve clearance issues on these heads if I shave off the bottom (just enough to true it up)?

Second, should I have the machine shop press in new valve guides since I have it apart this far? The ones in it seem ok.

The head itself is getting new valves, valve springs and valve seals. I have already lapped the valves on the 120 grit, I'm going to lap them again (220) after the head comes back shiny.

I'll add some pics to this thread soon.

No, you can actually take off a bit more to up the compression ratio. Check this: OZDAT.com Engine Design Utility

I would have them check everything, and if needed, replace.

Thanks nismo... I have everything off except the valve guides. I will have the shop do it. I don't think I will take anything off of the head except to true up the gasket face.

I think taking more off would open me up to some other issues that I don't want to have to address (like the cam and crank sprockets getting closer together), which I guess you could address with spacers under the cam towers... but I get over my head quickly.

Tower shims are only needed when removing more than .080" from orig deck. Shaving the head .020" is very common... I do .020" on every head. :)

If I could beg these questions a bit more. Whats the benefit of shaving .020" from the head, I know that will increase compression by making the combustion chambers smaller, but is there a 'weekend driver' performance gain associated with that? I'm more of a cruiser than a racer.

Second, when I was the the shop the guy asked me if I wanted him to resurface the valve seats, because when they replace the valve guides the seats need to be re-centered. I assumed at the time that I would be able to do this myself with a lapping tool, but now I'm not so sure.

I think I want to give him a call back and do the 0.020" and have him re-center the valve seats by machine before I lap them.

If you haven't clicked my above link yet, now is the time. :)

Lots of info on the web about increasing compression ratio & it's benefits (simple read: The Power Squeeze)

Since you are already paying the machine shop to cover the setup, time, and work costs to level the head, I'm sure it will be the same cost or only a few dollars more to shave it .020". In combination with a quality header and some easy intake work, it will make quite a difference.

Thanks again... So using the OZDAT site (along with some high school geometry refresher), the cylinder bore height on an L28 would change from 207.85 to 207.352mm (0.020" = 0.508 mm) ... and doing so would change the stock compression ratio of 8.257 to 8.613.

I've already done the intake work and have a new header... I will head back to the shop today (they asked me to bring in one of each valve). So that's about 1.5% overall performance gain.

Here's a few pics...

In picture #1, You can see in cylinder #1 that the coolant seeping into the combustion chamber from the failed head gasket had steamed the piston clean. If you look closely on the header side between #3 and #4 the broken head bolt. I was surprised I did not have more of them.

In picture #2, you can see the combustion side of the head and I have partially cleaned the first three cylinders after removing the valves. I'm pretty certain its the first time these surfaces have seen the light of day in 40 years. Again, cylinder #1 is cleanest so I started there (be gentle with myself)

Picture #3 is the cam side of the head, after pulling the cam, towers, rockers and lash pads, but before removing the valves. Again it's a bit dirty but generally looks ok. An interesting thing to note is that the oil "collars" (plenums?) that feed cam towers 1,2,3 stayed in the head, whereas their counterparts came out with the towers.

Picture #4 is the cam side of the head stripped down, including removing the valve seals and the two washers around each the valve guides that I'm sure have a cool name but I don't know it.

I have a series of small plastic bins that I labeled 1-12 to keep track of what parts came off which valve.

After the block comes back from the shop I will get to switch everything to righty-tighty, however I will have to figure out the broken bolt in the block.

Nothing like a hand tool problem that will ruin the car if you foul it up...

By the way, if anyone has the cover for that thermostat housing or can find me one I would be most appreciative in a 'couple cases of beer' sort of way. This seems to be a bit of a unicorn. I've checked MSA, black dragon, zcar source, ebay, amazon, a handful of Nissan dealerships... hell I even checked amazon. The only thing I found was a chrome version of the housing and the cover for a price I would only describe as prideful.

Update: I found one of these, so I am buying myself a couple cases of beer....

Got it back from the shop, stoked. I'd left the lifters in so they could only spray it off and not get it 100% clean. Still pretty stoked.

I was looking around for a nicer cover as well. Found an eBay listing that had just closed. Was wondering if it was you who got it ...

Hope the beer was good. :icon_super:

You should be stoked. Pretty soon you'll be Zoom Zoom (sorry Mazda).

If it was the really shiny one that included the housing and the cover, I was tempted but not at $150 bones.

I found one in North Phoenix, AZ... should get it next week (hopefully there's not a rattlesnake in it)

Thanks for the laugh. Already caught two this season, five days apart. One underneath the recycling bin (three feet from my workbench) and another crossing the pool deck.

I'm slowly reassembling the head now, have all the valved installed and put the cam towers back on.

I torqued the cam towers to 11.5 (per the FSM). There are a few posts out there that recommend up to 15 ft/lbs... I'm not sure why it would need to be tighter than the FSM.

Yes, best to slowly reassemble and take a few days rather than quickly reassemble and take a few weeks (and lots of $) to repair the damage.

Got it all assembled last night and it's ready to go back on the car. Now it needs to sit for a few days until I can get the broken bolt out of the block.

After two hours and 11 sizes of left-hand drill bits of questionable quality, I realize that I am a lucky bastard. Full speed ahead now...

That's a great photo. And yes, even though you were extremely patient and skillful, you are still a lucky bastard. Good for you.

How clean do I need to get the top of the block?

This is where I am now after almost an hour with degreaser and scotch brite pads. For the tops of the pistons I used solvent and a lift free rag.

Takes about 2 minutes with metal surfacing pads from Harbor Freight. I always clean to a bright shine... lots riding on that surface. :)

http://solutions.3m.com/3MContentRet...bute=ImageFile

I did just that, except for the two minutes part. It took me 3-4 hours.... spray degreaser, wait, scrub, repeat.

The head went back on without incident and I just have to put the rest of everything back together.

Holy cow, I cant believe it was three years ago that I did all this work.