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Neil Harvey Neil Harvey is offline
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Join Date: May 2004
Posts: 214
The topic of “head studs” is thwart with many opinions. Some valid and some steeped in historical “wife’s tales”.

Let me start with this. I believe many failures can be attributed to installation issues and the head stud takes the blame, always. In some cases, poorly written and vague installation instructions can be to blame. Understanding who you are selling to and their ability is important.

Many DIY engine assemblers assemble their engines in their home garage on the weekends, with a beer can and a book written on how to rebuild a Porsche engine, beside them. I know that better instructions and “why” certain installation procedures should be considered are important.

Years ago, we use to make our own studs and sell them. I think we had the only aftermarket studs sold by a engine company, other than fastener manufactures. I decided to stop selling studs to outside buyers because so often the issues were installation. We kept our studs inhouse for our own use. I also hated the vitriol spewed out by some suppliers about other competitor studs. One company that will remain nameless here, is particularly bad about this. Sell your studs because they of what they will provide not because you think others are junk!

Studs used in engines need to be used based upon the engines use, not because they are shiny and look nice. Are factory Porsche studs junk? No, but they did have issues with studs some years ago and since then the “all thread” types are very good. Are any of the other aftermarket studs sold today, no good. No would be my conclusion. I don’t think any are bad, some are better than others and some are over sold.

There was a lot of bad information put out there about ARP studs some time ago. We have used many sets of ARP studs over the years, in Porsche and in other engines. One engine we are involved in makes well over 3000 BHP and built with only ARP fasteners. In both the air cooled and water-cooled Porsche engines, selection of the right stud and proper installation is important. The ARP studs sold for Porsche engines have only been sold as stock replacement studs. I do not think anything sold by ARP to date has ever been built for high performance applications. However, they certainly have been used in high performance applications.

There are other very good studs sold. I have referenced the ARP stud as this stud got a bad wrap unfairly in my opinion.

Our use of any stud has shown up issues with installation. As an example, the water-cooled engines with single cylinder heads each side show up the installation needs required. You do an initial torque “seating” then go back in sequence and do an angle tightening. After the initial torque seating is complete, the nuts on the middle cylinders will be loose. Same can happen on air-cooled heads.

I have decided to write some assembly instructions that will go along with the new studs we are involved with. These studs are directed towards the high-performance world, not so much stock engine use. For stock use, the factory “all thread” stud is a proven stud. The cost is the issue now, so some of the good aftermarket studs can be a more cost-effective stud.

The topic of thread integrity has come up. Thread chasing taps verses thread cutting taps. A thread forming tap is supposed to straighten up the original thread back to its cut shape. Without removing material. Some care needs to be had here as some threads are so far out of shaped that these taps have been known to break. In the US, only “H or h” is used to classify threads, I think. Capital “H” is used to classify Internal threads and the numbers between 4 and 8 are used for internal threads. In most cases in the aerospace industry, 6 is a common value. So, any thread classification used in this level of engines, will be 6H.

When you stretch the head stud to apply clamping load to the cylinder head assy, the thread in the case gets pulled hard. The thread material is a lot weaker that the steel stud. Threads get deformed some. Therefore, it’s important not to bottom out the stud in the case thread. The stud needs to turn when the nut is tightened. Once the friction increases and overcomes the torque or angle pressure on the nut the stud will stop turning and the nut will tighten. If you bottom the stud and it cannot turn, all you will do is to pull the threads in the case out of shape.

The use of Loctite on the stud threads in the case is something that needs to be done with care. If you are going to be pulling the heads on and off in some quick succession, like Drag race engines, then go ahead and Loctite the studs in. Or if you will tighten the head nut before the Loctite cures, use it with care. But if the Loctite cures before you tighten the head nut, you are in the danger zone of having issues. Often the Loctite gives way and you feel and hear a snap as the Loctite gives way when you are tightening the head nut. The torque wrench clicks, or the angle gets all messed up and you have no idea where you are. My advice is, do not Loctite the studs. Use some grease and back the studs out at least turn. EPG sold by McMaster-Carr is a good grease.

Before you send out the heads for rewire, check the washer platforms for deformation. This is common. The platforms are not flat and now the washer contacts the platform only on the outer edges. Under the tightening of the nut, the washer collapses and there goes the accuracy again. In recent testing we have found a huge issue with accuracy, just in how the head washer contacts the head even with flat platforms. The new kit includes are new washer that addresses this problem.

How accurate is your torque wrench? Has it been calibrated recently. Do you leave it wound up or do you unwind the setting each time? Is it a quality wrench or is it a cheap brand? We take this part of the assembly very serious. To give you an idea, our Torque wrenches are checked every year. They cost over $2500.00 each and the values can be downloaded to files that show each fastener, its angle, its torque value, sequence, and a measured percentage of friction verse time to reach the desired torque and angle.

So, before you trash the heads studs, check your assembly procedures and do not take the assembly for granted.
Old 05-18-2018, 12:48 PM
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