RB20 fitment issues.

[10 inch Terror]

Ok I've recieved the new studs and fitted them to the casing and the cylinder still won't go fully home without a gentle tap. It's slightly better but still tight. The only thing I can do next is try it on an Italian case I have in my shed as I'm thinking that would be less likely to have the stud threads drilled in the wrong places. I'll let you all know how that goes tomorrow.

[Adam_Winstone]

Ok, getting closer then... that's a step in the right direction. What diameter are the new stud shafts?

I don't like the 'gentle tap' bit as this implies that there could still be a side loading (assuming that you have tried to fit just the cylinder to the casing mouth with no studs in place), which I don't really like. I've noted many times, on many builds, that you get this last tightness when:

a) really poor alignment issues (commonly the result of studs AND stud holes) means that the barrel doesn't want to go any deeper into the casing mouth than the shallow step roughly half down the spigot, or

b) pretty poor stud and stud hole alignment means the spigot goes beyond the spigot step mentioned above but then catches on the 5-10mm of exposed stud thread that protrudes out of the casing.

Does your tightness fall into either of the above categories? I can't stress just how important the removal of any exposed thread is beyond the gasket face. This small issue often makes the difference between pushing home by hand (perfect) or needing a 'gentle tap' to get it to seat (less than ideal and I'd address it).

If you've got no thread exposed at the base gasket face and the barrel still doesn't seat by hand (and you can confirm that your new studs are approx. 7.2mm shaft) then the stud hole positions or hole sizes are the most likely cause (noting that you'll have already got rid of the most likely cause; fat studs). You would then have the option of sending it back to the supplier and asking them to resolve the situation (refund, replacement, modification, other?) or of looking to identify what is still the cause of the problem. DON'T just go drilling out the stud holes though as you could break through to the Boyesen feeds (as mention previously), etc. If you do decide that you want to oversize holes then take all of the studs out and then replace them one by one until you find one or more wanting to pull the barrel off alignment. You often find that it is only one of the holes that is causing issue so drilling them all out is adding unnecessary risk!

Other things to consider, which you may have touched on earlier in the thread: have any of the casing threads been repaired (possibly changing position or alignment slightly), any welding to the casing than might have caused it to distort around the casing mouth, mag or mount areas? I assume that this 'gentle tap' is needed when you are fitting just the barrel to the casing, rather than fitting the head at the same time?

Give us some feedback on the above and then we'll be able to see what options are left to you (e.g. trying your other casing).

Adam

[warts]

Have you tried without any studs at all, to check for gross distortion round the flange or machining errors?
Then add a stud and check again, and repeat. May just give some insight....

Did your last attempt eliminate the "fat studs" as the cause?

[10 inch Terror]

Hi guys, with these studs there is 3.2mm of thread exposed at the gasket end so this I will address later. It was getting late last night or I would of done it then. The cylinder fits to the casing without studs fitted exactly as it should and will rotate by hand with no snagging. The new studs are 7.39mm so very slightly thinner than the MB ones. I'll try removing the extra thread as a 1st step and then try the Italian casing. Although it's a bit battered the crank mouth is in good nick. Cheers fellas I'll be back later...

[warts]

The plain portion of the stud does not need to be any thicker than the root diameter of the thread. If you can neck it down to that without causing any stress raisers, its fatigue life will be better.
The stud will stretch or even break at the smallest diameter, ie normally the root of the thread, and the thread, nearest the transition from thread to plain as the stress concentration is greater there, is the weakest link. The necking becomes the weaker part, but separates the stress concentration at the thread and the length of stud dealing with tensile loading. It also means most of the extension due to load can take place over the whole of the necked length, so reducing unit stress. Hence the usual claims for improved fatigue resistance.
The (slightly) smaller diameter should be way stronger than the stress applied. Unless made of butter (possibly not unknown with lammy spares).
Please correct if I am talking bol locks. (its been a long night).

[ducksta]

can these studs be frozen(cold treated) for extra strength

[Adam_Winstone]

Simply take the thread off so that it is flush with the shaft diameter, it shouldn't need to be (nor would I want it to be) any smaller a diameter than the standard shaft. Indeed, the idea of fatter studs would be a good idea, if it didn't impose such dramatic alignment constraints, so going smaller than standard should be considered a 'no no'.

It is a shame that there is such a long history of buggering around with studs and some dealers (one already mentioned, well done for trying to do something about it) have already identified the need for supplying the masses with a better product, however, to my knowledge there isn't a perfect product available. Currently, if you are unlucky with your combination of components then fat studs give alignment issues (even with standard barrels and casings) and I don't know of a standard (correct thin diameter) that has the perfect length of threaded section at the casing end. If some of the aftermarket studs were manufactured to the correct OEM diameter then they would be an ideal product... but until then they will continue to give people alignment issues.

One reason why aftermarket studs are made to the larger (problematic) diameter is that the larger size is a standard 'off the shelf' stocked item, meaning that it is cheap and readily available. It is extremely easy to cut a thread onto the ends of this stock round-bar and sell it on, however, getting the diameter reduced to the correct size adds cost.... but would make for a much better product that caused fewer alignment issues, regardless of kit, and (IMO) resulted in a much better product.

Let us know how you get on. Best of luck.

Adam

[Spanish Fly]

Without access to a lathe or a very steady hand I would try and tap the holes deeper, with a plug tap, before fettling the stud threads.

With regard to the stud diameter:
The probable reason why aftermarket studs are the full 8mm dia is because the threads are produced by thread cutting rather than rolling.
The thread rolling process displaces the material rather than cutting it away so the peaks of the threads are formed by the metal displaced to form the root.
For that reason the dia of the blank is required to be smaller than the finished thread outside diameter.
Generally (subject to the materials used) a rolled thread will be stronger than die cut thread.

SF

[Adam_Winstone]

10 inch... what news?

[10 inch Terror]

Hi Guys, sorry I've not had a chance to do anything else due to work commitments. I've a day off tomorrow, so, I'll have another go then. I'll keep you all posted Pete.