Vespa Grande Head Leak

Garrett Bell /
gbell
Feb 06, 2019 01:38AM

Hey all!

Setup:

1. Stock Vespa bottom end

2. 50 DR kit just installed

3. Polini head with lappped decomp

4. Stock

Just installed dr kit and did some leak tests only to find bubbles around the head, base gasket is good. Head gasket is tight on the studs so perhaps I need to trim the holes a bit but it’s not much and no major bends in the gasket after removing head.

Was curious if I should try to sand down the head or rtv the metal head gasket or run no head gasket or something else?

Any help is much appreciated!

-Garrett-

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Re: Vespa Grande Head Leak

Daniel '' /
deesal
Feb 06, 2019 02:38AM

You should lap the head, I use a diamond block and lapping compound, but you can use glass or a slab of granite and fine grit sandpaper. Permatex Motoseal or another anaerobic sealer like yamabond/hondabond. Siicone Rtv wont do work and it makes a mess.

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Re: Vespa Grande Head Leak

Graham Motzing /
grahammotzing
Feb 06, 2019 09:33AM

the other thing i've started doing on vespa builds is putting nuts and washers right up against the head, then the decomp stuff i attach with a second nut. you have to bend the decomp arm a little because the geometry changes, but its a much better way to put the head on.

i used to notice on the re-torque, they would loosen up a lot with the decomp parts all jammed up under there. when you re-torque it with the double nutted setup, you gotta take the two nuts back off, but they don't loosen up near as bad.

head and base gasket leaks are probably the biggest bugaboo on vespas, you're on the right track checking it before you put the engine back in the bike and sorting it out.

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Re: Vespa Grande Head Leak

Dirty30 Dillon /
hydra
Feb 06, 2019 10:02AM

Did you buy the DR used? An over-aggressive porter could blast through the third stud in the exhaust and cause all sorts of issues.

Barring that, I have lapped decomps and had them not work out.

But I will agree with Graham, putting a double nut on the decomp hardware removes that common problem with the bottom head bolt coming lose after heat cycling

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