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derekg
04-24-2009, 02:19 PM
Hi Folks,

Over the last year I've been working on a flycutter system to re-cut the rotor pad inside the Vespa 2 stroke engine cases. For those that have not seen a rotary valve 2 stroke, this pad is just below the carb and feeds fuel into the case when an opening in the crankshaft throw presents itself. The pad itself is "taller" than the case housing. The crank throw is semi-circular and when the "full" part of the crank comes around it closes off access to this pad and thus the fuel, working as an intake valve. We cleaned the Vespa cases, which are made out of a very porous aluminum. We welded the rotor pad and cut the welds down with the cutter system. Unfortunately, the welds also stirred up embedded oil in the porous cases and caused pits below the level of the final cut from the cutter. I decided an epoxy filler would work best (as Vespa used back in the 50's), but am not sure which fits the task. I tried the regular JB Weld in the tubes, but ended up just making a mess as the liquid nature caused it to spread and eventually run on the curved case surfaces. What I need is a firmer epoxy, maybe in stick form, that can handle the gas and 2 stroke oil mixture as well as the moderate temperatures inside a running Vespa.

Any suggestions would be greatly appreciated. BTW, no cutter system for the rotor pads has existing for Vespa since the 1950s. I feel like I am an inch away from finalizing it, but as Indiana Jones said, "that's usually when the bottom falls out".

Thanks, Derek

Evan
04-24-2009, 02:46 PM
The trouble with epoxy is that while it may have high viscosity it also has very low surface tension, which makes it a good glue. It also makes it run although slowly, with even the slightest amount of slope. I would be tempted to experiment by mixing powdered graphite with epoxy resin before adding the hardener to make a thick paste. This should give it good self lubricating properties as well as a degree of temperature resistance. You can buy small tubes of powdered graphite for use as a dry lock lubricant.

darryl
04-24-2009, 02:58 PM
I've used an epoxy called PC-7 which has impressed me with its holding power on aluminum. It's advertised as non-sag or drip, and has high chemical resistance to fuel oil, gasoline, mild acids, etc. On the can is says do not expose cured epoxy to temperatures above 82 C. Maybe this will kill it for your application, maybe not.

It claims high tack, but I find that you need to give it some time in place to develop an initial hold. Nothing wrong with covering the repair with poly, then keeping it all in place with a backing of some kind til the PC-7 sets. Poly will peel right off once it's set. Sets in a couple hours, but needs a full day or two to be strong, like most two-parts.

Ian B
04-24-2009, 03:25 PM
If I understand correctly, you're trying to epoxy around the inner wall of a cylindrical chamber.

Rather than doing the whole thing in one go, can you set the crankcase so that one part of the cylinder is horizontal, epoxy a narrow strip, let gravity do its job and let it set, then turn the cases a bit, epoxy the next strip and so on? You'll end up with a sort of polygon, which I guess you'd then remachine to remove the excess epoxy.

Doing this will let you warm the cases, making the epoxy bond better and hopefully giving a better end result.

You mentioned oil in the pores - won't this affect the bond of the epoxy, or is all the oil out now?

Ian

crancshafter
04-24-2009, 03:42 PM
Hi Folks,

Over the last year I've been working on a flycutter system to re-cut the rotor pad inside the Vespa 2 stroke engine cases. For those that have not seen a rotary valve 2 stroke, this pad is just below the carb and feeds fuel into the case when an opening in the crankshaft throw presents itself. The pad itself is "taller" than the case housing. The crank throw is semi-circular and when the "full" part of the crank comes around it closes off access to this pad and thus the fuel, working as an intake valve. We cleaned the Vespa cases, which are made out of a very porous aluminum. We welded the rotor pad and cut the welds down with the cutter system. Unfortunately, the welds also stirred up embedded oil in the porous cases and caused pits below the level of the final cut from the cutter. I decided an epoxy filler would work best (as Vespa used back in the 50's), but am not sure which fits the task. I tried the regular JB Weld in the tubes, but ended up just making a mess as the liquid nature caused it to spread and eventually run on the curved case surfaces. What I need is a firmer epoxy, maybe in stick form, that can handle the gas and 2 stroke oil mixture as well as the moderate temperatures inside a running Vespa.

Any suggestions would be greatly appreciated. BTW, no cutter system for the rotor pads has existing for Vespa since the 1950s. I feel like I am an inch away from finalizing it, but as Indiana Jones said, "that's usually when the bottom falls out".

Thanks, Derek

Derek.
I can recomend the products from this company, have used "tons" of their products in repair-works for more than 30 years:http://www.belzona.com/prod1k.aspx
BTW I have no interest in this company
Regards CS

Langanobob
04-24-2009, 04:12 PM
Just how big are these pits? If, as it sounds, they're big enough that epoxy wants to run out of them, maybe you should bite the bullet and reweld and re-machine? Any residual oil must be gone by now after the first welding job.

About epoxy, as has been mentioned, I'd suggest calling Belzona and/or Devcon and get their recommendation. Guaranteed to cost a lot more than J-B Weld :)

Off-topic, I haven't messed with two-strokes for a long time, and I'm curious what are the advantages of a rotary valve system vs reed valves?

Bob Farr
04-24-2009, 06:22 PM
The Belzona products look good, but I suggest sticking with something that flows well before curing. I've had luck with epoxy on outside curves or in holes by building a damn around the area. Modeling clay, tape, whatever works to keep your epoxy flowing in the desired direction until it cures. The excess can be machined or filed off afterwards.

Edit to add: I recently used a two-part epoxy product by Loctite called "Metal/Concrete Epoxy," which I found at Home Depot. It flowed well when first mixed, and set in about five minutes. The package claims it is "resistant to solvents" but you may want to test it first. It filed and sanded beautifully, and it adhered very well to this 2024-T3 aluminum. However, I have only used it on a non-structural part as a fillet-filler over top of stitch welds. Heat is not an issue on this part, either:

http://i58.photobucket.com/albums/g272/frankenglide/Fairingmount26.jpg

Tinkerer
04-24-2009, 11:58 PM
Lab Metal will do the trick.

http://www.neverseezproducts.com/labmetal.htm

macona
04-25-2009, 04:03 AM
Yeah, Labmetal does work nice.

Also Devcon Aluminum Putty: http://www.ellsworth.com/display/productdetail.html?productid=808

jerryweier
04-25-2009, 08:46 AM
I had similar situation with trying to tig weld a cast aluminum oil filter cover......after several attempts , I elected to use an aluminum brazing rod from harbor freight which worked quite well.....works with a propane torch......Jerry





Pack of 8 Alumiweld Aluminum Welding Rods

WELD ALUMINUM WITH A PROPANE TORCH!

Now you can weld aluminum with any low temperature torch--even a propane torch! Amazing technology makes a stronger weld than the parent material without flux or other welding material. Alumiweld makes a stronger weld than the parent material.

* Stronger than aluminum
* Harder than mild steel
* Non-toxic
* Low working temperature (730 F)
* No flux or fumes
* Excellent corrosion resistance
* Minimize parent material distortion during welding


ITEM 44810

GKman
04-25-2009, 10:31 AM
If you have enough oil contamination to affect welds, I would expect problems with epoxy bonding.

I haven't seen a vespa rotary valve but I have seen others and know a little about the principal. Many 2 strokes run and run well without a rotary valve (or reed valve) because of the port placement in the cylinder wall, loop scavaging etc. I never thought of a tight seal at the rotary valve like the poppet valves of a four stroke. I think of a rotary valve (or reed valve) as more of a shield to reduce burble back through the carb at certain rpm's. In which case I don't see pitting in welds as a problem.

Anyone agree?

derekm
04-26-2009, 02:01 PM
I suggest trying to clean the cases using acetone and a vacuum pump.
Bolt the and seal the cases to a plate that has a connection for the vacuum pump. Wipe acetone into the out side of the casing and turn on the pump. The acetone will be drawn into the porosity and clean out the contaminants.
if you cant bolt it to a plate, another way is to put the entire part into a vacuum chamber and by pumping to very low pressures remove all of the contaminant withany significant vapour pressure.