Head stud length and base gasket
 

Hi all,

As per another thread, I am in the process of rebuilding my 1980 911SC RoW:

- 190K Kilometres, engine never opened before, rebuilding due to many broken lower headstuds, did not spilt the case.

- Had the heads rebuild by a pro. Replaced all guides, stems, seals, valves and flycut 0.007" from sealing surfaces to clean up very minor damage caused by driving with broken studs.

Questions:

1. Before I removed the old head studs, I measured height to be 5.248" (~133mm). I read that spec for this should be 135mm, but considering this was the stock measurement, and there is 0.007" shaved off the heads, I am thinking that I'll set the new (steel) studs at 133mm height. Agree?

2. Considering that there was only 0.007" (0.1778mm) flycut from the heads, I am thinking that the standard (0.25mm) base gaskets should be ok. Agree?

Thanks in advance and happy motoring!

The original studs on my SC motor are set at 133mm as well. Not sure why the spec is 135.

I would do a trial assembly with the .25 base gaskets and measure the deck height. No reason to take any chances since some parts have been machined.

Many thanks for the reply. Did you replace the studs and, if so, what height did you set them at?

Yes, a trial assembly is a good idea. Do you use the solder technique, or just measure along the outside edge of the pistons at TDC?

I replaced the bottom row of studs. I set them at around 133mm as well. The end of the stud was fully engaged with the nuts, so all seemed good.

I used both the solder and measured the edge with my caliper. The caliper is the way to go, since the edge of the piston will be the closest to the head. That is the measurement to use for deck clearance.

Thanks Gordon! 133mm it is then + red thread-locker :)

I'll do a test-fit soon and measure deck clearance using the caliper method; all is stock, no change to pistons, cylinders etc, just 0.007" fly-cut off the heads.

Head stud length
 

Setting the studs a couple of mm short won't hurt anything. The main thing is to not get them too long or the head nuts won't tighten properly because the end of the stud will push the allen wrench out of the nut. Keep in mind that 2mm is only about .080"! Good luck with the rebuild!

The way to deal with stud length is to ditch those barrel nuts and use reduced head flange nuts. Supertech studs come with them, but they are easy to purchase in either coarse or fine thread and use on any of our engines. No worries any more about getting height just right.

The thread locker is for the studs, right, not the nuts? I'd not use it on the nuts.

Thanks Fred, and others for reply. I'm setting them all at 133mm, with red thread locker on the stud ends (into the engine case).

Many (most) of the new studs are easily installed by hand / fingers, more loose than the original studs. Is this normal??

Good advice. Unless you are 100% committed to a completely stock rebuild, this is the way to go. This will allow you to focus your time and energy on getting the studs into the case correctly, without bottoming them out.

Hi again Gordon,

What value should be used for the deck clearance measurement?

Thanks,

My deck height was 1 mm or 0.040", which is probably the safe minimum.
http://forums.pelicanparts.com/uploa...1493603525.jpg

Head Stud Depth
 

As Walt mentioned - head stud depth really only matters if you are using the original head stud barrel nuts.

- For barrel nuts: you need the correct head stud depth / exposed threads to ensure that you are able to torque the barrel nuts to spec without the head stud going through the barrel nut to the extent that it comes in contact with the allen tool as you are tightening the barrel nut (i.e. pushing against your allen key, and reducing the depth that the allen key / socket seats in the barrel nut).


- For aftermarket studs that uses a hex nut, head stud depth doesn't really matter, although when I rebuilt my motor using ARP studs, I ensured that they were relatively close to the standard specc for head stud length / depth.

Based on your description, it sounds like you are re-using the barrel nuts, so yes - getting the studs installed with close to 135mm (stud exposed) is rather important to ensure you your head stud threads are in contact with a sufficient amount of the barrel nut threads (and to ensure as you torque the barrel nuts, it isn't in contact with the top of the head studs).

Meanwhile - the safe bet is setting the head studs slightly deeper (133mm to 130mm) to ensure that your allen key doesn't contact the stud as you tighten), vice going more shallow.

As for your comment regarding setting the studs - yes, I was able to set the depth of mine by screwing them in by hand; others took a little more effort and assistance (mostly dependent upon how clean the threads were).

Difficult to describe - but it sounds like you are asking the right questions...


Good luck,

Gordo