A question on joining crank cases

[Canuck750]

Not a Guzzi but a Benelli two stroke single.

Benelli specified a thin paper gasket to seal the vertically split crank cases of the 175cc two stroke single I am working on. The gasket set I have is probably 40+ years old and the gaskets came in a sealed bag with the gaskets folded over to save space. The gaskets are dried out and have shrunk making the large centre case joint gasket useless to me.

I have rebuilt Japanese single cylinder two strokes that had no gasket and relied on 'Yamabond' or similar to seal the cases.

Is it acceptable to use a 'Yamabond' type sealant instead of a paper gasket?

Thanks
Jim

[LesP]

I see no reason why not if the gasket thickness was not needed... You could iron the gaskets between paper and dampen and they might be OK (unless there was a oil passage between the halves hence a gasket)


With Yama or ThreeBond you could use a single (flat type) dental floss on the joint face + the sealant... I think they used to use silk in the old days.

[Canuck750]

With Yama or ThreeBond you could use a single (flat type) dental floss on the joint face + the sealant... I think they used to use silk in the old days.

Thanks Les,

I have never heard of a string of floss or silk along the joint face, what would the purpose be?

[RinkRat II]

   If the gasket in question is Vellumoid or vegetable fibre they can be brought back to life by a bath in warm water. ( sometimes)

         Paul B

[Murray]

3m three bond, is a bit pricey.

[Perazzimx14]

Thanks Les,

I have never heard of a string of floss or silk along the joint face, what would the purpose be?

To maintain the thickness of the original gasket.

[LesP]

Thanks Les,

I have never heard of a string of floss or silk along the joint face, what would the purpose be?

On two strokes (being an air pump of sorts) it was an added barrier to (air) leakage and perhaps if fuel puddled in the case and tried to seep through the joint ( before modern sealants)  ... iirc silk thread is still a recommendation in some aviation along with sealant.
Paper gasket wiped with Loctite 515 would work also.

Straight Yama/Three bond (grey) 1100 series ? would be fine with no raised area's around any fastener holes (light countersink if needed)

#

The grey mastics have a good reputation, you could use Wellseal applied to both surfaces and left to stand before assembly or blue Hylomar also.
Some depends on the condition of the joint faces where YamaBond would be the best bet if any nicks etc.

[Antietam Classic Cycle]

   If the gasket in question is Vellumoid or vegetable fibre they can be brought back to life by a bath in warm water. ( sometimes)

         Paul B

This ^^. The gasket might just need a good misting with water. I do it with a spray bottle. Lay the gasket on a blue shop towel, spray until it's covered, turn over, repeat. Apply while still damp. I use Permatex 300 with that type of gasket.

On the Morini's vertically split cases, I just use Hondabond 4.

[Canuck750]

Thanks for the advice fellas, I will use a case joint mastic, I think I have a tube of the Honda Bond.

I had soaked the gasket in warm water but it was too deformed from being folded and dried out, its a big gasket with a inner circular section to seal the cylinder crankcase from the balance of the casting with many holes for bolts to pass through, I just could not get it too expand enough to mate to the case.

[Moparnut72]

When I worked in the radial aircraft engine shop we redid them exactly the way they were done by the original manufacturer. One manufacturer used thread between two halves of a the main case. We did it the same way with silk thread and Permatex Ultra Copper. We never had a leak. It was a real job keeping the thread in place.

For what it is worth, I would NOT use Yamabond. That stuff is awful, we had to charge customers extra to clean cases and other parts of that stuff. I was also terrified of breaking a $2,000 case trying to separate the case halves glued together with that crap. I assume the Honda stuff is similar. If you are interested I can explain a neat way to make perfect gaskets.
kk

[rschrum]

+1 Hondabond 4

[moto-uno]

  One of the benefits of Yamabond , was that you didn't have to take it apart again  , as they say YMMV , maybe I was just lucky using
it for so many years .  Peter

[Testarossa]

The general problem with silcone-type gasket goop is that if a blob squeezes into the crancase it can migrate into an oil pickup. If it were me I'd make a new paper gasket:
https://www.youtube.com/watch?v=Cv05ziVYQOw

Buy sheets or rolls of gasket paper online in a variety of thicknesses and sizes.

[Canuck750]

I used HondaBond after thoroughly cleaning the joint surface. trying to trace a paper gasket given the size and complexity of the surface to seal was not something I wanted to attempt



Bottom end reassembled, waiting for the cylinder overbore to be done

Italians sure know how to design a pretty engine

I did make a new cylinder base gasket though from stock sheet I have, easy enough to trace and punch out a simple gasket like that one.

[moto-uno]

 ^ Nice looking job , and to assuage another gentleman's remarks , those 2 strokes don't have oil passages ,
even the Yamaha's pumped their injector oil externally ( if my memory serves me ) . However when silicone
seal was new , I remember slews of Honda 350's having their cams ruined because of exactly what you said !
 Peter

[Canuck750]

^ Nice looking job , and to assuage another gentleman's remarks , those 2 strokes don't have oil passages ,
even the Yamaha's pumped their injector oil externally ( if my memory serves me ) . However when silicone
seal was new , I remember slews of Honda 350's having their cams ruined because of exactly what you said !
 Peter

Yep the 'beauty' of a simple single two stroke like this is it doesn't even have an injection system to the carb manifold like on a Yamaha of similar vintage, good old fashioned pre-mix of fuel and oil, a bit of a PITA but dead simple. One thing I do like about the transmission is the gear selector system, it uses a pair of springs to center and return the shift mechanism, no heavy spring to break.



The clutch uses a single spring like on the Aermacchi 250 / 350, very simple but a wee bit of a challenge to compress while refitting the holding nut







I always find it interesting to see how different designers tackled the same problem.

[Testarossa]

That's very cool, and I missed the fact it's a 2-stroke. Nice job!

I once rebuilt a Laverda that had a blocked oil passage. The factory was notorious for overapplying "gorilla snot."