Modern Vespa

This is the second time in 2 months that the same cylinder stud has stripped its threads under the head nut. WTF is going on?

Some facts and observations...
This is a PX150 ('05) and it normally uses 7mm x 140mm studs. I torque the nuts carefully in an x-pattern to 11 lb-ft using a calibrated wrench.
This started happening after the head was replaced by HRA with a non-Piaggio one that I'm thinking might be a bit thicker since the nuts seem to torque down closer to the end of the threads than they used to. During rebuild I did measure each stud for length after install and this particular one did seat about 1 thread deeper into the case than the others, consequently it's about 1 thread shorter than the others. That said, all nuts do completely clear the threads when torqued, but they are close to the end.

Just guessing, but should I be using the longer version of studs? Would that help? I'm not sure why this keeps happening on this particular one. Anyone have any suggestions?

Hard to see,is the nut striped onto the threads?or is nut striping threads off bolt?

The nut is stripping the threads off the stud, same as last time. What has me baffled is why the same one twice in a row?

Maybe stubs weren't treated right,still soft. Those studs should take more than 11.Right?

I would certainly think so. My motor experience is with aftermarket Harley-Davidson, but I have 8 years in a specialty shop. The issue we'd consistently encounter in high performance big bore applications is the studs pulling the threads out of the alloy cases, as that's the weak link in an otherwise steel chain. I too, suspect stud quality.

I wasn't sure if...
a) the compression was too high (150psi) for these 7mm studs, or
b) there simply weren't enough threads at the end past the nut.
(I'm not sure what difference that makes though when the nut is fully threaded on unless the threads near the end just happened to be weaker than those further down).

I'll get another stud and nut ordered and try this a third time but I'm inclined to think something's not right.

i had similar problems once.
i just figured
a) the studs are soft to prevent damage to the case, allowing stripping,
b) the nuts fit loosely, making cross-threading likely
c) i suck as a mechanic, making cross threading likely
d) all of the above

Hello

I thought the studs were 8mm, however having said have you replaced all four studs and that is the one that strips or is it just that one that has been replaced?

Grumpy

All four were replaced in early Spring. The upper flywheel one stripped (just like this) in late-Spring. Thinking it must've been faulty, I replaced it and the nut shortly afterwards.

The replacement one has just done the same thing. That's why I think this is weird. Even if it were a bunch of bad studs, what's the likelihood of a new one that happens to be in the same location getting stripped the same way by another new nut? The case threads all seem snug and there's no sign of galling.

Hello

That is odd, why if all the studs are at the same torque that one keeps stripping, you will have to replace it and you can then play with the one you have removed to check on hardness etc.

Grumpy

I have no idea what's going on. I do know this location sits about 1 thread deeper in the case, so there's even less room at the end when the nut is on. Maybe it's when I double nut it to install it and that somehow weakens it - next one I'll double nut at the bottom of the threads, just in case. I don't know, grasping at crazy ideas. If it does it again, I'll try one of the 150mm studs from SIP, I guess.

I have seen Bajaj studs, nuts, and bolts do this before. maybe just a bad stud? just a guess.

Jim have you seen the M9 über hard ones at SIP? I know their Hardened cross is a bad idea, but maybe the studs are good? Don't know personally.. I haven't tried them.

I have seen that they have 'performance' ones from DRT, et al, that are also longer. But they also list 2 different stock ones, a Piaggio one and an 'economy' one that may be of different quality (or may have the same OEM for all I know).

I'd be a bit nervous using a hardened one in the case threads, but if this happens one more time I'll have to consider some of those options.

maybe it's DRT that I was thinking.. one of those.. Yeah.. I'm wasn't sure about 'em either.. I'm not in need, but saw 'em and thought I'd mention... I'll have to ask Al (Aviator) about the Hard v Softer metal in a situation like this, and see what he says.. he knows a lot about that kind of stuff.

I think the only thing that would cause that kind of damage to the threads is cross-threading the nut. It's really easy to cross thread that nut with those cooling fins in the way. 11 foot pounds is toward the low end of the torque range. I always do 13 foot pounds.

I suppose it's possible that my new CDI wrench is out of whack, despite what last year's certificate says, but if so, why would I keep stripping just this one. The only thing I can think of that makes this corner unique is the fact that the stud screws into the case a little further, making it a little shorter than the others, and it's almost too short as the nut barely clears the end when on. As soon as the replacement gets here I'm going to try not screwing it in all the way and using threadlock to hold it at the same height as the other three.

I'll post a picture when reassembled.

Yeah. It's best not to screw them in all the way. There will still be plenty of threads in the engine case.

Or pack with saw dust

In my opinion, they're a bit on the short side. Screwing them (the short end) all the way in makes the most sense to me.

Yeah. I've been working on Vespas for 18 years or so and I still have to mess with them for a but till they're right.

all the way in is best,with thread usage to the max ,best strength with more thread in case.Then again you only have about 5-6 threads on nut.Try torque it by hand-feel.Last 2 kits i did with out a torque wrench.

Hello

In my humble opinion, providing you have one thread poking through the nut,there is no difference in holding force than if you have ten threads poking through.
It is just odd that the same stud keeps failing.

Grumpy

Hello

Just as an aside 11 ft lbs is just a tad over 15.5 nm which is in the centre of the range.

Grumpy

I've been having the same problem with a P200. New studs stripping. As far as I can tell I am doing everything right.

With the Pinasco head ther is plenty of stud left. Start nut with fingers, deep socket spin down by hand, wrench tight on star, torque to 6 ft/lbs star, torque to 10 ft/lbs star, try to torque to 14 ft/lbs and two stripped.

Personally I think there are just some bad studs around. I'm going to order more and also use long "spacer" nuts to distribute the torque.

There is only one solution.... JBWeld

lol nice one

thumbs up!

Shitty quality repro studs.

That's what I think. I put in 4 new studs and 2 stripped at 14 ft/lbs. Ordered hardened from SIP.

For P200 blocks, I use the BGM Lammy stud kit. Excellent quality.

I'll try them next time. needed a few other things from SIP. It just pisses me off because I put 4 new studs in. Oh well...That's Vintage!

The studs are "soft" and "crappy quality" for a reason - engineered that way by Vespa. You are dealing with two different metals and a part that expands and contracts with engine temperature. Vespa has made the weakest link the nuts and then the studs, to prevent case thread damage. Don't try to "improve" upon their design - you will be sorry, After all better than a half century of experience and engineering speaks for itself - unparalleled.

Having battled the same problem with my 7mm studs, I noticed that the stud that kept eating threads was also deforming the flat washer worse than the other 3. Close inspection revealed that the machined surfaces on the head where the washers rested had all gotten a bit concaved over time and the one in particular was worse than the others.
Suspcious that this was resulting in uneven stress being applied on the torqued but soft nut, I went looking for some grade-8 flat washers but couldn't find any in 7mm, but I did find some extra thick grade-8 ones in 1/4" size that fit perfectly without binding, so I'm trying those right now. My hope is that using a washer that stays flatter will help. Other options would be to have the fastener surfaces on the head milled flat again and stay with hardened washers, or maybe upgrade to 8mm studs.

If this is the cause though, this might explain why it seems to be a problem with certain studs on just some bikes. It's the head.

poor quality parts, happened to me with studs I got from beedspeed, initially I thought it was me so ordered new set and those were crap as well, finally ordered sip serie pro at 4 times the price, well impressed with those quality item, they've even got a notch cut so you can get a 6mm spanner on so you don't have to double nut

You might have nailed there, with the washer nut combo settling in to the head, this used to happen on the triumph twins and caused all sorts of leaks etc, chred with a hard flat washer.

Grumpy.

I had some studs from one of the standard vendors that stripped just like that. I feel for the OP...you figure..:OK...as long as I'm doing this I'll change out the 50 year old studs" You torque them properly and they you get the ..."Oh F**K!!!"

I used the first oversized SIP hardened studs and they have worked great.

I guess youve cured it by now!

I also think if the washer was bending then the head needs some work.
Washers are very important to help obtain and maintain the correct torque.

As a rule of thumb, i was always taught a nut should be fixed down with at least 1/2 the diameter of the thread size protruding past the nut.
The very ends of the studs dont really have enough thread on them to be of any use.
Along with the fact that the nuts are very likely stronger than the softer studs as they are likely treated to make them shiny!

Plus there are a load of cheap parts out there anyway.

A 'soft' (untreated mild steel 4.6) 8mm stud would not take much more than 14Nm before the threads go.