Discussion and questions related to the course Practical Engine Building
i bought custom head studs for my engine but the top and the bottom have the same thread pitch size. Does it have any negative effect? i know its causing the nuts coming loose easier. should i use these head studs ?
The studs dimension is m12x1.75 ,nuts threat 1.75 ,The material is H11 tool steel
in my experience i would be wanting to see a finer thread at the top to give a smoother and more even torque load
if the engine is not turboed and you not seeking large hp they may be ok
With clean, lubricated threads, it shouldn't make any difference in smoothness when torquing the fastener - where it WILL make a difference is a fine thread has, for the same torque, a greater tensile loading - clamping force. That said, you may find the recommended torque with these studs is significantly higher than stock, to take advantage of the greater strength - are the nuts and washers also a higher tensile steel and hardened, respectively, to aid clamping?
H11 tool steel will have very little stretch under load, so unless running very high boost levels, you should be perfectly OK.
this is the reason Im confused and want to ask in forum. I never saw a head stud like this before. Generally they have a fin threat on the top. i will expect high hp on this engine and i dont want to create a problem because of that . they recommended a final torque value of 12.6 kg which is not so high and they use same material on nuts and washer. what I also would like to know is, if there is any way to test the studs before using them?
The important property that you are interested in for a head stud is tension. Tension that can be achieved by a stud is limited by the tensile strength of the material and the smallest cross-sectional area of the part. In the case of the design that you show in your photos the thread is the smallest cross-sectional area (bad design but not uncommon), so even if you had a finer thread on one end that would not allow you to achieve higher tension, it is always going to be limited by the smallest cross-section which in this case would always be the coarser thread.
The main reason for using a finer thread at one end is because "torque" is very poor way to measure tension. A finer thread does give slightly less error but it is still pretty bad.
The torque required to acheive a specific tension is highly variable and thread pitch, lubricant, surface roughness and thread accuracy have a large influence. If head stud tension is critical for your application then it is best to determine the correct torque (or angle is better in many cases) by measuring stretch in a dummy assembly, just like you would for a rod bolt.
Obviously you must also consider the effect of the head stud tension on other parts as well. For instance if your studs were tensioned to 50kN when the engine was bored/honed, then it is no use tensioning to 70kN during final assembly as it will pull the bores out of round.
correct, the studs are as strong as their weakest place. it will be quite difficult to measure the head studs stretch gauge but not impossible .Unfortunately, i don't have a torque plate for this engine. Using recommended torque value he didn't mention any bore changes, but I am planning to install the cylinder head and remove the crank and measure the bore from the bottom end before assembly. İf there have any fail i have to make custom torque plate and sleeve the engine again.
What engine is it, many modern vehicles use an open, or semi-open deck where there is no material connecting the bore material to the block where the bolts/studs are, this means a bore/hone plate is irrelevant and not needed.
Even with closed decks, the further the fasteners are from the bore, and the thicker the deck, the less any distortion will be.
The engine mercedes m120 6.0L v12 engine also pagani use this engine long years, attached down the engine deck pictures and block studs hole info that you can have more idea the stock engine cylinder bores are alusil before, but we sleeve it with cast iron without torque plate.
Those have enough material, and are far enough away, that you 'should' have negligible block distortion - Mercedes have been making VERY high end engines for well over 100 years, you should have it right by now ;-).
Ah, from what I have been able to find, in limited time, is the H11 is ~ equivalant to the ARP L-19 material, with a UTS of ~280,000 PSI UTS.
Checking the ARP catalogue - https://arpcatalog.com/ - page 24 (page 22 was where your last image came from) has some torque values for their fasteners with a 220,000 PSI steel being 125 lbs.ft, which would suggest 160 lbs.ft?
This page for their SS fasteners suggest 98 lbs.ft for a 180, 000 PSI steel, which suggests 152 lbs.ft.
As has been mentioned, even rolled threads on a full diameter stud will have the main failure point at the last thread - I'm VERY surprised they aren't a waisted design as, with that material, the weakness will be the block threads, not the fastener.
TBH, I would be seriously considering passing on those studs and going for some waisted ones with a fine thread on the nut end, with torque spec's from their manufacturer, especially when you consider the relatively low cost compared to the full build?
As an aside, when using very high tensile loads, it isn't unusual to have the washer under the nut/head actually sink into the head, so I'd suggest making sure the washer is as large a diameter as practical, preferably a thick, hardened steel alloy one.