Changing the head gasket is simple and straight forward, helpful hints:
* Get yourself a service manual for a start
* tag/bag nuts and wires etc so that you dont have to remember where all the bits go ...
* If you are just replacing the gasket, you can get by without removing the intake/exhaust manifolds. However you will need to be very careful to inspect the bolt holes and clean the gasket surfaces. It's easier to do this if you disconnect the manifolds and remove the head completely.
These are important
* Take the head bolts out in the opposite of the torqueing pattern
Torque Patterns
* If you don't have a torque wrench, buy or borrow one. An inexpensive one will work, don't need a fancy ratchet type. If you don't use a torque wrench, there's a good chance the gasket will faill after driving it for a while, or the bolts break if you tighten too much.
* As mentioned below, get the head, block surface bolts and bolt holes clean
* Check for straightness of the head (discussed below)
* don't use gasket sealer (according to my repair book and most gasket makers ...) unless your qasket maker specifically recommends it
* The oil bolt goes in Number One position (has a T mark, hollow head or is shaped differently)
* Generally, for any engine, tighten the middle bolts first, then work your way outside in a circular pattern, see the pattern photos.
* Loosen/tighten in three steps, my book doesn't say exactly what, so you can guess at the exact amounts, for example:
* Tighten all 2/3 torque
* Tighten all 80% torque
* Final-tighten full torque
Torque Specification
(engine cold)
engine torque lb ft torque Mkg comments
A12 33-36 ft-lb 4.5-5.0 Mkg (to engine # A12-470084)
A12 40-43 ft-lb 5.5-6.0 Mkg (from engine # A12-470085)
A13 51-54
A14 51-54 ft-lb 7.0-7.5 Mkg
A15 51-54 ft-lb 7.0-7.5 Mkg
Clean the bolts and bolt holes
Check when reassembling an A series is that the bolts holding the head down are lightly oiled. Also clean out the bolt holes as they easily fill up with crap and the bolt bottoms out. If the head has been machined a lot, ensure the bolts screw if far enough before putting the head back on.
Get the head, block surface bolts and bolt holes clean (no grease, no tiny remnants of the old gasket). Use a scraper and maybe a little ether spray to chase it off. Make sure nothing gets in the cylinders or oil passages, and clean out the bolt holes when done. Oil or water in the bolt holes will prevent proper torqueing. You can use a straw to blow out the bolt holes. I use an old head bolt, with a slot filed along one side, as a poor-mans "tap". You can also use a can of compressed air.
Everything must be absoulutly clean, if there is any of the old gasket left on any surface, or if you use any kind of sealer on the head gasket, this will cause standard-type gaskets to fail again.Cleaning and lightly oiling the threads will allow the bolts to torque to spec. instead of binding up and giving a false torque reading.
Special Bolt
As for head bolts, the A series engine uses 2 different ones, the one that goes in the center right (distributor side) is different from the other 9. the oiling system for the valve train (rockerarms) feeds oil through this hole, putting the wrong bolt in that hole will restrict oil flow to the rockerarms and valves. Under high rpm conditions if the oil is restricted to the valve train it will cause excessive wear and possably nonrepairable damage. I have seen some so bad that the rockerarm shaft had grooves worn into it deep enough that the valves couldn't be adjusted.
Do I Need to Mill the Head Flat?
Head gaskets require flat surfaces on both the block and the head to seal properly, the block is heavy enough that it usually won't warp, heads on the other hand will and do warp very easily. The factory allowable tollarance for warp is 0.003 in. (not very much), any thing more than this will require that the head be surfaced to make it flat again. This is no big deal to have done, just a little work to remove the manifolds so it can be done.
On the other hand, the problem is that every shop I've ever talked to says "oh that's not right, you gotta mill 'em every time or you'll warp the gasket ... and it's only $ to do it - it's cheap insurance". What a bunch of baloney -- as if the factory doesn't know anything. For or a stock or mild engine, it is more important to get the block and head scraped clean (no old-gasket residue) and get the oil off the surface before putting the new gasket on. I use a little starting fluid (ether) to get the oil off. As rgrinder explains, torque the bolts in pattern to the correct torque steps (use a torque wrench) and it'll be just fine.
If your engine did blow the head gasket, use a quality straight edge and a feeler gauge to measure head warp (if any). A good straight edge is a precision device, so you might have to round up an old mechanic friend or have a shop check it. It usually doesn't warp 3/1000 of an inch even after blowing a gasket. But it can, so it is important to check.
Check for straightness of the head (discussed below). Often it won't need machining, even after a blown gasket, but you want to be sure. Place a quality straightedge across the head surface and use a feeler gauge to check any warp. The spec is that it can be up to 0.004 in. (0.1 mm). If it's over this, get the surface machined flat (milled). Keep in mind that the "normal" warp is up to 0.05 mm, so yes even new heads aren't perfectly flat. A good straight edge is a precision device, so you might have to round up an old mechanic friend or have a shop check it.
Other Tightening Torque specs
Rocker shaft bracket bolts 14-18 ft lbs (2.0-2.5 Mkg)
Manifold nuts 6.5-10 ft lbs (0.-1.4 Mkg)
Rocker cover bolts just past finger tight. Don't overtighten or the flange will deform and oil will leak out. You can always tighten a little more later, but you can't un-tighten to fix a deformed flange
Don't forget to adjust the valves!
Changing the head gasket will undoubtedly change the gap, even if you somehow get it back together without loosening them.
As written by: ddgonzal at http://www.datsun1200.com" onclick="window.open(this.href);return false;
