Posted on Leave a comment

HPI Savage Petrol Conversion – Fuel & Silicone – Chemical Compatibility

While I was already well aware of the effects of petrol on silicone products – the stuff swells up & dissolves over a very short period of time, which makes it an unsuitable material for seals in a petrol fuel system.

Fuel Tank Cap Seal
Fuel Tank Cap Seal

I wasn’t aware the O-Ring on the fuel tank cap of the Savage is silicone, as can be seen in the image above it has swelled up to much larger than it’s original size. It’s supposed to sit in the groove on the cap & fit into the filler neck when closed.
This was only from a couple of hours of petrol exposure, now the seal is such an ill fit that the cap will not close properly.

The solution here is to replace the ring with a Viton O-Ring, 2.5mm cross section, 23mm ID. I assume the fuel tank is made of polypropylene – this should stand up fine to the new fuel.

Another concern was the O-Rings on the carburettor needles, however these seem to be made of a petrol-resistant material already & are showing no signs of deterioration after 24+ hours of fuel immersion.
The O-Rings that seal the engine backplate to the crankcase also seem to be working fine with the new fuel.

Another silicone part on the engine is the exhaust coupling, between the back of the cylinder & the silencer, I’m not aware of a suitable replacement as yet, although as it will mainly be exposed to the combustion products & not raw fuel, it may just survive the task.

Exhaust Coupling
Exhaust Coupling

The extra heat from burning petrol in one of these engines may also put a lot of stress on this component, if it eventually fails I may attempt a replacement with automotive hose – time will tell on this one.

Fuel Bottle
Fuel Bottle

I’m also not sure of the plastic that standard fuel bottles are made from – their resin identification number is 7, so it could be any special plastic, but I’m guessing it’s Nylon.
However according to the spec sheet for Nylon, it’s chemically compatible with petrol – yet the plastic appears to be getting softer with exposure, so it may be a special blend designed specifically for glow fuel.


Besides these small glitches, the engine is running well on it’s newly assigned diet of petrol, I’m currently running an 18:1 mix of petrol to oil (250ml oil to 4.5L of petrol), this seems to be providing more than adequate lubrication. While it smokes like a chimney, plenty of unburned oil is making it out of the exhaust, so the engine’s internals should have a liberal coating.
I’m yet to actually run the model out in open space so I can start tuning the mixture, but bench tests are promising.

More to come!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.