Is this spark plug thermal paste ok

Dow Corning 340 is apparently an acceptable substitute. One probably shouldn't stray too far from the original spec Rotax stuff. The heat conduction via the paste is part of the overall engineering: too much has a potential of being just as bad as too little.

Kevin and Jeffrey, Thanks for great info and links :)

Slight hitch with the didgi key nz site is its just a site and it wil come out of the states taking weeks and costing alot. Im looking to see if the digi key product is in NZ anywhere...so far no one has it in stock,

The Dow Corning 340 is easy to find but no tech sheet I can find so far gives a temperature rating..... EDIT  just found a tech sheet with bleed etc.. % at 200 deg c so that should fit the bill

Ill keep trying to find something here in NZ

Might look further into the HTSP ans see if that higher height transfer efficency is acutally a problem...

Dominic

Wacker Paste 12 - still waiting to hear back from Australian supplier, Barnes Products https://www.barnes.com.au/

MG Chemicals 860, 150 gram - Can be had from;

Mectronics https://mektronics.com.au  $40.16

Element 14 https://www.electronicsonline.net.au/directories/element14-724452676  $34.09

XON World https://www.x-on.com.au/ $???

Dow Corning 340

Element 14 https://au.element14.com/dowsil-formerly-dow-corning/340/heat-sink-compound-tube-100g-340/dp/101685 100 gm $26.70 200 gms $46.53.

Note: Element 14 have pdf data/specification sheet on almost all products that they supply (include the Dow Corning 340)

          All prices in Australian dollars inc GST (tax)

As far as I know all of the international Digi-Key sites are serviced from their warehouse in the USA. When I have ordered from the UK site, the orders have been delivered in about 3 days and all of the taxes and duty are covered in the price shown on the website. If you spend over a certain amount there is no additional postage charge.

I don't know how long shipping to NZ takes but I would imagine that they would want it to be within days rather than weeks.

Jeffry Stetson wrote:

Dow Corning 340 is apparently an acceptable substitute. One probably shouldn't stray too far from the original spec Rotax stuff. The heat conduction via the paste is part of the overall engineering: too much has a potential of being just as bad as too little.

That's an interesting point, although I have to wonder how much of an impact it has. In my cabinet of random thermal pastes, I found some $$$ silver-based silicone heat transfer paste, and it's about 6-7x more performant than the Wecker stuff. 

Thermal paste makes a big difference, in terms of backing off a cliff, but going from 0.8W/m-K to 5W/m-K is unlikely to have as much an impact as simply having a thermal paste at all (vs. bare metal). And this coupled with the fact that our engines are not really temperature stabilized, so the spark plug could just as easily be heat sunk to 50C metal as 150C metal. That range is far wider than what the paste can do.

In general, I'm really curious why Rotax chose this route. It seems an incremental solution, but I suppose that if you're already running the coolest sparkplugs with the smallest gap there's really not a lot more of dials you can twist and knobs you can pull to further reduce heat. But if that's the case, then the better pastes ought to give improved results.