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1966 Vespa 150 Super

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Two strokes are interesting technical beasts.

On the advice of those wise in their ways, tonight I cut a couple of mill of the top of my shiny new performance cylinder,


shaved the head down a couple of mill,


and made a 2 mill base packer (shim) to space the cylinder and head further from the block


In the morning I’ll find out if I did it right.

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Shim design worked out nicely.


Mark one is not usually  keeper, but will be this time.


To get the squish down, I ended up recessing the barrel,


And lowering the base of the head, giving it a lip to locate it on the jug.  Head seen here ready to re-measure squish.


Which is now at 0.6mm to 0.7mm all around.


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I see what you are doing....... but not quite sure why (except for the squish bit).  Why raise the barrel? Two-strokes as a group are not my favourite IC engine - though to be fair most of my exposure comes from the very low end - lawnmowers, chainsaws and strimmers.  Vermin engines!

On the squish front - you know about Somender Singh? https://somender-singh.com/patent/patentpage.html


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It's a cheapo upgrade kit.  The basepacker moves the port timings so more power becomes available (closer to the expensive ones...).  I'm still working my way through the theory of it off, trusting in the experts at the Vespa equivalent of Sideways.   Basics are here http://www.dragonfly75.com/motorbike/portdurations.html

Mr Singh's document doesn't mention how his ridges are going to create hotspots, and therefore have him pinging like crazy...


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All buttoned up and ready to install, but still failing the leak test.

No seals or outside seams were leaking, eventually used an old trick Ward once showed me.

3/16 fuel hose makes a handy stethoscope.


There’s a nasty hiss inside the case. Gaskets blown.

Time to open it up, 


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Frustrating!  Casings not quite flat?  Need to tighten bolts in a different order?


PS, why've you got your whiskers on - it's not winter!

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There’s a known weak spot.  

This one.


That’s one mm of slippage, and it’s toast.

So I cleaned it all down again.  


I cleaned it a lot - especially the recalcitrant flange.


(Heineken helped make the work more beerable!)

This time, I followed some instructions from the net.  First, I put the sealant on the outer casing, not the gasket, and built up a bridge under the weak spot, but being careful to keep the corner and wall of the flange clean of sealant.


Then I put the gasket onto the tacky sealant, centered all the bolt holes, snugged it in tight to the walls, and ran a broad blunt screwdriver blade over it to smooth it all down and squeeze out the excess.


(Note the sealant bridge support)

Icleaned off the excess, and tidied up the edges.

After a thorough clean I put a smeared layer of sealant on the main case.


Then married them up oh so carefully, and bolted them up in order, sneaking the torques up slowly and in order till they were all at 11ft/lbs.

The head and cylinder both got a smear of Permatex,


I mated them up on the bench, pushed down hard, then separated them and cleaned off all the excess, leaving a light little smear.

I gave the casing matingsurface a light cover since there’s no gasket with the spacer, being careful to keep the ports clear,


Then buttoned up the head, making sure I got no permatex in the cylinder or on the rings or piston. 


New hardware all around, and head sneaked slowly up to 14ft/lbs.

Then, rather than pressure test it right away, I gave it an hour to settle, and re-toqued it all round.

Then I razor bladed and cleaned off any excess permatex I’d squeezed out (I like a clean and tidy engine!).

It’s time to pressure test it, but also time to eat.  So I had some dinner, and posted this.  I’m about to go out and see if it’s airtight.  Not sure what more I could do if it’s not!


ps. Yep the beard is back - I’m a glutton for punishment over summer.  Lost my razor in Vietnam in November, and I’’m fascinated to see how far it will go....

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1 hour ago, GT6MK3 said:

Yep the beard is back - I’m a glutton for punishment over summer.  Lost my razor in Vietnam in November, and I’’m fascinated to see how far it will go....

All the way to the ground.........:biggrin:  You'll likely get fed up with it before then though - or receive an ultimatum from Senior Management.  If I grew that much fur I'd be out on my ear......

Gasket..... looks like it damn well ought to work.  Choice of gasket goo and curing time is likely critical.  You need some of that stuff Triumph used - the one stuff that needs a hammer and sharp chisel to scrape off..... Wellseal?

Another thought..... does that casing actually see positive internal pressure? @Cause I'd have thought it was actually vacuum - and the casing has a ledge to support the gasket in that direction......... Vacuum test better?


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Not sure right now if it’s under pressure or vacuum, but right now it both sucks and blows...

Went back out at midnight, and it still had a slow leak somewhere.   Rude words were used.

Only promosing thing is that even in the dead of night, it didn’t have an audible hiss in the hose-o-scope.

So maybe it’s the cylinder/casing seal this time.

I went to bed pissed off.

Autopsy on the head this morning

Head contact looks ok, the smear of  permatex pushed wide on compression, leaving the contact ring pretty much clean.


Ditto the cylinder to spacer join,


Permatex has squashed out to leave a clean mating surface,  spacer is clean too


But, on the space to casing join, there’s a small web of permatex left on both surfaces on the bottom of the flywheel side.



So I decided to try another approach, and get some RTV in there.

After another cleaning session, I went Ultra Copper all round, using the put on a bead and push to squeeze method.



The Cylinder head I was pretty generous with on the outside of the ring, but I cleaned up the inside to ensure nothing got in the bore.



Everything was then put together, hand tightened, and left to cure for an hour.

Then I torqued it all down and went to work.

Came home tonight hoping that I’d find it holding air at last.

Yeah, naaaah.

300mmHg becomes 200 in 50 seconds, than 100 seventy seconds after that, then nil after about 3 more minutes.

I can still hear nothing in the case..

There’s no sign of a leak on the fly side, including around the studs and around the cylinder join.


There’s nothing showing around the top of the cylinder join, the case join or the test kit join.


Ditto the base of the cylinder and the exhaust plug.


Clutch side shows no results


Testing in the cylinder/ head gap was hard, but flooding it with bubbles showed nothing


New plan needed.

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Went and made a few bits to help solve this on Alan’s mill today.

Then I clocked up and milled out a spare clutch cover ready for a much needed bigger clutch


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This thing may break me.

Im just gonna leave these here for today.

I’ll come back and explain if it ever bloody holds air.








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I keep blowing through gaskets in the weakspot, so I bought some fancy silicone impregnated ones.

Took a before shot prior to toque down


It didn't help - I still had some blow by, so I hooked up my compressor via my car leakdown kit, would as low as it would go.


That let me confirm the leak was audibly in the case, so off came the fly side again

The gasket shows signs of definite compression


and certainly shows this in the danger area


I'm getting sick of trying gaskets, so I've painted the little bugger in ultra copperto see is I can find where my low spot is, and see if that will fill the leak.


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Look on the bright side, at least its only one cylinder not 6!

feeling some of your pain, having wasted two weeks trying to move some oil where it doesn't want to go, and stop it going where it does!


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Must be a low spot in the casing?  Hard to measure with certainty and also to deal with.  There's some pretty serious instant gasket poop out there as used by the big OEMs in their engine building.  My employers sell small pumps to Jaguar LandRover, Nissan & others which are held in a robot arm and used to draw beads of gasket on mating surfaces.  If the pumps are not cleaned properly, they die and get sent back to us to deal with.  We lob them straight in the bin - impossible to clean it all out.


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