Head bolt tap size

[Namerow]

There's nothing like a good mystery story...

As luck would have it, I have in my workshop a fully-reconditioned, stripped-down L24 block sitting on an engine stand.  I also have a depth gauge!  What I discovered is quite interesting...

The block has 7 head-bolt holes along each side.

The depth of all of the holes along the passenger (right) side of the block is 26.5mm

Things are much more interesting along the driver (left) side.  Numbering from front to rear, the hole depths are as follows:

1. 32.0mm

2. 29.0mm

3. 25.5mm

4. 32.0mm

5. 26.5mm

6. 32.0mm

7. 28.5mm

These measurements should be considered accurate to +/- 0.5mm.  Nevertheless, there's a substantial variation along the passenger-side.

Why is this?  I'd have to look at the head casting (not convenient).  However, I expect that height of the bosses for the head bolts have variations as well.  You also have to take into account that there are three different lengths of head bolts (the parts manual calls them 'S', 'M', and 'L').  They may have different threaded lengths, too, but I don't have a set conveniently available to check this.

It seems to me that -- properly assembled -- each head bolt should end up with relatively full engagement of its threaded length.  If you get their locations of the S, M, and L bolts mixed up, you may end end up with problems.  I can't find any info in the L24 FSM.  It only offers this cheerful note: 'Three different types of bolts are use (So be careful when in installing).' 

My Haynes manual does have a bolt diagram --  although it says there are only 2 different bolt lengths - 'Short' (S) and 'Long' (L).  With that noted, Haynes says:

Passenger Side:  all 'S'

Driver's Side - from front to rear:  L - L - S - L - S - L - L

Maybe Tom Monroe's book, Rebuilding Your Nissan/Datsun L24 Engine' offers more detailed info.  Unfortunately, I've misplaced my copy.

As a first step, I suggest that you use a depth gauge (or a 'calibrated' piece of wire) to ensure that all of the hole depths in your block are about the same as my measured depths.  I'm pretty sure that they will be.  It seems inconceivable that just the tip of a bolt would break off at the bottom of its hole.  If one of these bolts snaps off, it's going to happen at the top of the threaded part, not half-way or three-quarters of the way down, so you should be able to clearly see the top of the broken bolt sitting at or slightly beneath the top deck of the block. 

BTW, all of the bolt holes are blind.  That is, none are open at the bottom to some other water/oil passageway.

 

Edited August 31, 2019 by Namerow

[Reptoid Overlords]

Thank you for your research!  I will take this into account for sure.  I have blown some compressed air into the bolt holes on the block to ensure all FOD is out of the way, shone a light into the holes and verified to the best of my ability that the holes are free and clear all the way to their stops.  I will also measure the threaded lengths of my new set of head bolts to verify any variations vs the old ones.  My new set is a Nissan turbo set, ordered through a local dealer.  I'm under the impression that these bolts have an updated torque spec of 5 or so more ft-lb than the earlier ones?   It would be good to know if the thread lengths are the same. I am going by Tom Monroe's book, it does not appear to elaborate, however I will read ahead a bit to see for sure once I head out to the garage today. 

I will also measure the bolts I removed at the time of disassembly.  If I were more seasoned, I would be able to tell if this engine has been rebuilt, as all internal components seemed pretty clean for the age and all seals were annoyingly sealed with gasket shellac.  The freeze plugs looked brand new. 

[Namerow]

My depth gauge 'pointer' is actually a metal strip measuring about 3/16" across, with a square profile at the end.  As someone else has pointed out, the bottoms of the holes are conical rather than square.  That means that the tip of my depth gauge would have sat slightly short of the very bottom of the hole at its centre.  If you use a piece of wire as a depth gauge, you measurements may come out 1 or 2mm longer than mine.

[Zed Head]

Run the bolts in to their holes without the head and measure the distance from the bolt head to the block.  If it's less than head thickness it's as good as it needs to be.  In manufacturing, precision and accuracy cost money.  There's no purpose to having all of the bolt holes the same depth so that spec would not be set that way.  

[Captain Obvious]

I've only ever seen two length head bolts*, and the positions are pretty easy to figure out... Long ones go through the cam towers and the short ones go everywhere else.

Five long ones for five cam towers.

*I did see in the 1972 FSM where they referenced three bolt lengths. So that's either a feature of the early cars, or a typo. I've never worked on one that early, so I can't verify the lengths.

 

[Reptoid Overlords]

I ran all the aforementioned measurements, and could find no reason to suspect I had a problem. Cylinder head to deck thickness was in line with bolt head to deck distance(with washers taken into account), so I moved forward.

Head bolts lubed and torqued in 20 ft lb increments to 65 ft lbs with no binding or complaining.

Made it as far as just before you get the front cover on and begin installing and setting the oil pump and shaft. Now to put a beer in my hand.

Thanks to all that helped. It's much appreciated.

Sent from my Coolpad 3310A using Tapatalk

[Captain Obvious]

10 hours ago, Reptoid Overlords said:

Head bolts lubed and torqued in 20 ft lb increments to 65 ft lbs with no binding or complaining. Now to put a beer in my hand.

Excellent! Sounds like great progress.

I don't know if it's really necessary, but I re-torqued my head bolts after about 100 miles or so.

Edited September 1, 2019 by Captain Obvious

[Reptoid Overlords]

11 hours ago, Captain Obvious said:

I don't know if it's really necessary, but I re-torqued my head bolts after about 100 miles or so.

I ended up gong with a Fel-Pro gasket.  They claim no re-torque is necessary.  I've had mostly good experiences with Fel-Pro, but no experience with their head gaskets.  A lot of people I trust say they have no problems.  Let's hope that's the case. 

The only issue I encountered yesterday during head install was that some oil I used to lube the valve train found its way to the base of the head and contaminated the gasket behind cylinder #6.  I cleaned it really good with some isopropyl alcohol and made sure It didn't happen again before I started getting bolts in place. 

[Captain Obvious]

11 hours ago, Reptoid Overlords said:

A lot of people I trust say they have no problems.  Let's hope that's the case.

Agreed. Hopefully no revisit.

I used an OEM Nissan gasket because after all the problems I had with my motor, the last thing I wanted to do was mess around with the head gasket again. Not saying that's any better or worse than what you used, but just that "if it's good enough for the dealer"...

And there is a note in the FSM about re-torqueing, with the assumption that your using an OEM gasket of course.

I figured what could possibly go wrong. Go wrong. Go wrong.