Brake cleaner is just a flammable as avgas (for the purposes of this discussion) and avgas is always right at hand.
In my garage I use brake cleaner and in the hangar I use avgas.
I would use avgas and obviously let it dry before starting the engine. Should dry in very little time.
Lots of people in maintenance use avgas but don’t talk about it 
100LL avgas has at least three big advantages: 1) it is readily available near aircraft, 2) it does not harm 99% of items in the engine compartment, and 3) it dissolves and suspends oils and grit with little residue.
The big bad one is it is harmful for the environment and a very harmful spray to breathe. One of my mechanics died of a terrible throat cancer after a lifetime of breathing avgas vapours at work. I never spray it without one of these:
Yes it is also very flammable, more so in spray form, and warrants commensurate precautions, but that is the case with most of the alternatives.
I limit use of mogas to outside the engine compartment and where there is no fuel tank or other polysulfide sealant or acrylic plastics, gaskets o-rings or sealants, mainly to avoid degrading any materials.
The ideal one is mineral turpentine or white spirit #1 CAS 64742-82-1 which does away with the worst part of the environmental impact, while maintaining advantages #2 and #3 of avgas. Just don’t let it sit on paint, as some types of paint might get damaged.
In practical terms, and for one-time use, avgas 100LL is the safest, with a proper breathing apparatus, skin protection and ventilation and brush application where possible. For recurrent use, get white spirit.IN general, as long as you know the source, some engine oil inside the engine compartment is 99% not harmful. I disagree that it will not spray the windshield. I once bought that t-shirt , near the end of this post You might get some, but it should be quite limited. Some rubber bungees for the landing gear or otherwise might be negatively impacted (I dont think that is the case for the TB20)
Antonio wrote:
The big bad one is it is harmful for the environment and a very harmful spray to breathe.
Thank you! I was already wondering why nobody mentioned this with a single word. Same goes for Diesel and I wonder why anyone would use that despite tho god-awful smell.
Diesel is horrible. It leaves a nasty stain – like car petrol (nowadays “mogas”== car petrol, usually). 100LL is a great solvent which evaporates with virtually no residue. IPA is better still and safer but not as aggressive a solvent as 100LL; I use IPA to wipe down all tools after a service.
IPA is however harmful to polysulphide sealants (mainly used in integral fuel tanks and pressurized airframes) and some rubber materials used in aircraft systems, so it must be used carefully.
For sure one must be very careful about what one puts in the fuel tanks
Socata spec max 2% alcohol in the fuel. AFAIK they use the standard PRC type sealants.
But the OP just wants to wash down the engine and the inside of the cowling.
