Modern Vespa

Here's the situation: my P2 engine had been running quite nicely for a while; only issue had been some slight bogging when cold, which routinely cleared up after a quarter mile. No biggie. But the bogging started lasting longer, up to the point when I was headed out for a countryside ride and had to return home after about 15 miles because in the low to mid rpm range the engine seemed to want to stall. I only managed to get the engine starting again by messing with the idle screw.

I checked the ignition: pick up and LT coil gave good readings. Switched carbs: same issue.

I then decided to try a leak down test to see if there was some transfer of engine oil into the combustion chamber, and after checking that the oil seals, the cylinder and its head, and the casing were airtight, I found this:

The stud whose threads the air is leaking from is one of the 4 surrounding the crankshaft: the top right once the rotor is removed. That stud is separated from the crank housing with a paper gasket and liquid sealant (reinzosil). The stud itself has no play, and was glued with loctite 271. There should be no way in hell that air could go past the torqued-down cases, the gasket + liquid gasket, and the loctited stud, but here we are. Any ideas how this is even possible?

I ended up filling the remaining empty space at the butt of the stud with JB Weld, and it no longer leaks from that particular area. But there's still a leak somewhere that takes the pressure from 300 to 260 mmHg in 10 minutes.

Possible… air is escaping from the cylinder, along the surface of the crankcase through the stud..

Doesn't seem likely, they don't connect. And the cylinder studs are loctited as well...

Lol

Loctite is Cyanoacrylate, which cracks under heat/stress. Any heat or stress on that part of the motor?

I'm going to go with "leak on the sealing surface between cylinder and stud hole, exiting through the base of the stud because the stud is sealed by RTV.

Loctite 271 is a threadlocker (the strong type), and afaik is meant for these kinds of applications. Not talking about super glue. Are we talking about the same thing?

It's mostly methyl methacrylate.

Similar, but not the same, you're right.

I usually use 272 for these applications, which has higher heat resistance (450°F); I wasn't paying attention when I restocked with the 271, which I just found out has a lower resistance at 360°F.

Still, none of these studs are secured from the factory; there's no reason they should allow air to flow through the threads, especially with threadlocker, which is supposed to make threads airtight as well.

It says it's a thread sealer as well as locker, but the only place I've ever used it where I *needed* a seal was on a couple of carb studs when I'd extended the intake timings and broke through. I wound up having to JBWeld those spots to get them to seal.

Maybe I had the wrong loctite.

I had the same leak on the other side bolt hole. Blue Loctite and sealant.
I'm guessing you applied sealant outside the bolt (vs inside/around it) like I did.

Btw that location doesn't get close to 360F.

Do you know for sure that those studs weren't airtight? I would have assumed some strong threadlocker would have done the job....

The strange thing is that there's no leak on that side of the casing, even with just a washer, a grower washer, and a nut to "seal" it. Cool, the lower resistance range had me second-guessing my choice there...