Modern Vespa

I have this pesky leak I can't seem to get rid of. Cylinder is new with about 300 miles on it ran perfect before I split the cases to change selector rod. Re-assembled cases, cylinder and head. Did a leak test thinking I damaged the crank seals during assembly. Seals are fine but this leak on the side where the cylinder meets the cases I can't seem to get rid of. Leak is at 4 psi test pressure. Any ideas??????

What sort of base gaskets and/or sealer are you using there?

Do you have a picture of the cylinder base and case deck?

Did you forget to trim the tab off the case gasket so it's not sitting flat?

1. Just the metal style cylinder base gasket. I never used any sealer before and had good results so I thought it was not needed. If it is needed, any suggestions?
2. I will take apart the motor and post pictures of the cylinder base and case deck.
3. What case gasket tab are you referring too?

This tab…

You have to use some sort of sealer to get a air tight engine…

You might be able to get away with it with grease and a paper gasket if the cases are in great condition. But any imperfections in the case will create a leak.

Any I usually use high temp gasket maker and that works great… no blocking or degradation with fuel and 2T oil.

You don't need to split the cases for that. Just remove the head, lift up the jug and paint some semi-liquid gasket sealer on both sides of that aluminum gasket with a q-tip. If you make a mess, don't worry. You just need coverage.

Man o man. I have that ticket stubb and sure wouldn't recommend that show.

+1 to every reply above.

They did the keystrokin' so I don't have to.

And a double reminder that you don't need to even drop the rear shock bolt if you don't want to. You should be able to do the entire thing the q-tip by just removing the head.

I do understand the purpose of a gasket and sealer in this instance. Machining imperfections. Head to cylinder not so much for some odd reason. I would love to know why.
The engineer in me wants to know what happened so I don't repeat the incident. The sealer, in my mind is addressing the symptoms not the original problem.
Head, Cylinder and cases mated perfectly for months now. I removed the motor then head, cylinder and split the cases. Changed the incorrect selector rod, new case gasket. Assembled the cases new cylinder gasket, added the cylinder then head. Torqued in the correct sequence and 4 to 5 steps until I reached 11 foot pounds. Decided to do a leak test and discovered the cylinder to case leak. What did I do wrong? I am concerned that any sealer will squeeze out and might plug those small lube holes for the main crank bearings. Also, I could not get the studs loose to remove the cylinder so I had to remove the motor.

I think you got lucky with assembly last time.

The original crankcase halves shift around considerably with only 1 fixing dowel at the bottom.

Stock engines are definitely less sensitive to air leaks, but you'll want to have a good healthy running motor air tight.

It's recommended that you use sealant in the cylinder head too. Even if it has an o ring, I will add the extra layer of protection.

If you're worried about bearing oil passages being blocked, you're using too much sealant.

Should be applied as thin as possible (thinner than paper), but without seeing the aluminium of the crankcase. And used on both mating surfaces.

Cylinder studs should unscrew easily, if loctite has been used, heat will help.

Chandlerman requested pics of the Cylinder base and Case deck. So here they are. Note some rust were the leak is from the soap bubbles.

That's definitely going to leak. Surprised you didn't have any problems before replacing the selector rod.

Surfaces look way too uneven. Looks like it's been apart a fair few times..

Even if the cylinder is brand new. It might have a hard time matching up to be flat.

Ok 108 Thanks for the input
What surfaces are uneven?
Cylinder and or Cases? And what can I do about it?

The case gasket surface is scratched pretty severely.

Yeah the cases, the case machining surface seems to have taken a bit of a beating.

Scratches at the top will leak.

The red area is the milling from the factory, you can see at the bottom in green what looks like a low spot. The milling should've covered that area. The green patch on the left looks, you can see some of the milling swirls, but also it's patchy.

You'll have to take off the cylinder studs, take it to a machine shop to get decked, or what I would do is a piece of 240 grit on a piece of glass and sand the surface till it's even.

Blue machinist layout fluid would help, if not a sharpie covering the surface is better than nothing to see flat spots.

OK 108
That's exactly what I need to know. THANKS !!
I found those cases in a bone pile last year. My previous cases rotary pad was damaged beyond repair by the last owner, so no telling the history of abuse. I have only split them once.
I will do the 240 grit with glass method, however, any suggestions on what size of glass to use. I have a piece that's 3/4" X 1" X 3" seems a bit short to me. Also since I was having trouble removing the studs, where is the best place to heat? The stud or the cases?
I have access to the blue machinist fluid so that should help my cause.
I did notice when fitting the cylinder, it did rock a bit side to side. Since the top left bolt was the first to be tightened, it pulled the cylinder to that side and the opposite side leaked. Things make more sense to me now.
I'm guessing the cylinder will be ok.

The best way is to lap it on a piece of plate glass, like 108 said.

You're going to need a piece of relatively thick glass. 1/2" would be ideal, but you can get away with thinner, and a foot or so square to do the deck. A glass tabletop is generally going to be your best option, but take what you can get.

The other way people have solved this problem is to take an old cast iron cylinder and some valve grinding paste. smear the paste on the deck, then put the cylinder in the opening and spin the cylinder until the deck is even.

And in case it wasn't obvious, you're going to want to bolt the cases together before you do either option.

As to why you (think you) got away with this the first time, did you re-use a metal crush gasket? They crush down to better address unevenness of the deck, but using them lets the magic out so they're strictly single use.

Did not use any of the old gaskets. I learned my lesson with that way back in the 70's

I usually use an approx 10"x10" piece of glass to flatten old cylinders and cylinder heads which you put on the table and swirl the part in your hand.

With the crankcase (because of the size), I'd do it the other way carefully or you'll be holding onto broken glass.

With the studs, heat the cases around the base of the stud, a squirt of wd40 and the double nut trick with a 30cm breaker bar works well.

Don't forget to plug the engine case so aluminum sanding dust doesn't get in there

He'll have to remove the crank for this one, but taping over the bearings and seals would be a good plan since I'm guessing OP doesn't want to have to replace those again.

If you do a fair amount of making things flat, you might consider a machinist's surface plate. Grade B is fine and it doesn't have to be huge. There are a lot of cheap ones.

Oh yea. Duh…. That cod rod ain't going all the way back in. I should have known better considering I literally just replaced a cylinder on my 62 Allstate

Dratt !!!!!!!!!
Things just got a bit harder, I didn't think about the connecting rod sticking out.
As for removing the studs, where to put the heat since they don't budge. Stud or cases?
I hope not to make anymore things flat ever, ever again.

The best way to do this is to take it to a machinist and have them deck the gasket surface. This is 100%. I do this service but i recommend finding someone local before shipping it out to anyone. the setup on this takes the most time out of anything. expect to pay something like $40-80; Californification prices may be more because that's how the do.

Anything worth doing is worth doing right.

OK Vespa People, I solved the mystery !!!
I decked the cases using a piece of glass 3/4" thick. Still did not stop the leak. I then looked closely at the cylinder. See pics. The cylinder has a ridge where it was not machined correctly. 80% of the surface showed a clean level surface but that one area with the leak has a ridge that contacted the cases first creating a gap at the port area. The leak was at the port outside of the ridge. Perhaps the reason I did not have an issue before could have been a better and/or thicker cylinder gasket. When I split the cases to change the selector rod, I used a new gasket set and have had nothing but this leak since then.