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I thought I'd try and make a hebel brick displacer piston. My thoughts being it was relatively light and insulative. Surprisingly it worked very well. I question how well the hot end will fare but I'm thinking of coating that with JB high temperature paste. I've only run it for a few minutes so far. Nonetheless its quite encouraging. I'd be interested in the experience of others as I doubt I'm the first person to have tried this.

Nut wrote: ↑Sun Feb 25, 2024 4:31 am I thought I'd try and make a hebel brick displacer piston. My thoughts being it was relatively light and insulative. Surprisingly it worked very well. I question how well the hot end will fare but I'm thinking of coating that with JB high temperature paste. I've only run it for a few minutes so far. Nonetheless its quite encouraging. I'd be interested in the experience of others as I doubt I'm the first person to have tried this.

First I've heard of it. Australia? Not sure it is something that could be found here (USA).

Recently I was trying to use some automotive muffler patch paste as a kind of high temperature "glue".

To test it, I got it red hot with a propane torch, but it had not dried so the paste kind of just "boiled" and bubbled into a kind of rock hard fire proof foam which, heated by the torch, eventually hardened and dried and became red hot. Also it ended up being very light weight. Pretty misshapen though.

I have found that with foamed cement, generally, it seems to take the heat great, at first, but after a few times being reheated it eventually starts to crack and crumble.

Maybe a coating of something like the muffler patch paste would help protect it.

I can't say anything about Hebel brick specifically. I'd certainly like to sample some though.

I only just a few days ago "discovered" that I could make a kind of lightweight fireproof foam out of muffler patch paste. I may make a video. Seems like it could be pretty good for making small high temperature displacers.

I might try cooking some in the microwave kiln inside a plaster mold or something, see if it will take the shape of the mold.

Not sure if it could be milled. Seems really tough. Intended to stick to and plug holes in a car muffler rattling around under a car through rough roads, high heat and rain etc and hold up under such extreme conditions.

Not a recommendation, I haven't really tried it yet, but seems worth experimenting with.

Hebel is a brand name. It is (as you have tried) foamed/aerated cement. I agree with the crumbling potential which is why I thought of a heat paste coating. The first iteration I impregnated with superglue. Although the cold end was fine the hot end vaporised the glue.

I also tried the kneaded high temperature putty but it didn't harden. I'm not sure whether that was because I didnt knead it thoroughly enough or whether it may have been out of date. Again the concept has potential but the putty is quite expensive.

I'm in the process of trying VincentG's suggestion of molding a piston using JB Weld epoxy.

I have learnt a lot in this last week. My take away is that if you can get the hot end of the displacer sorted (ie thermal breaks and insulative piston) and minimise overall friction you're 90% of the way to having a working engine.

What size does this displacer need to be?

The displacer is 16mm x 40mm diameter. Which I have now just realised doesn't line up with recommended ratios. Maybe the designer who drew the plans had a reason.

I covered the Hebel piston in a muffler paste tonight. I heated it until the paste hardened then lightly sanded to smooth everything back to dimension. I ran the engine for approximately 30 minutes and it didn't miss a beat. Once it's cooled I'll strip the displacer to check how the piston has held up. Fingers crossed, it may solve the potential crumbling problem.