GX390 Flywheel Nut Question

jbradley · Dec 22, 2019

I recently purchased an electric start conversion kit for the GX390 engine on my BCS 948. My shoulders are complaining about the amount of torque that they have to pull against to get it started. I don't have a socket head that fits the flywheel nut. Does anyone know what size it is? I tried to measure it with calipers, but I can't get them close enough.

flyerdan · Dec 22, 2019

Looking at several parts diagrams for different version of that engine, they all indicate that it's 16mm.

jbradley · Dec 23, 2019

flyerdan said:
Looking at several parts diagrams for different version of that engine, they all indicate that it's 16mm.

I found that also, but that's the threaded portion. The head is larger.

flyerdan · Dec 23, 2019

So it is, 16 x1.50mm. Since an 8 x 1.25 nut takes a 12mm wrench, it would make sense that this one should be about a 24.
Do you not have any metric sockets, or just a spotty set that omits a bunch of sizes? You could probably run a crescent wrench or some needle nose vice grips down until they were snug and then measure that with the calipers.

jbradley · Dec 25, 2019

It is a 23mm.
I knew it was less that 25, because my 1" socket was too large, and my metric sockets only went through 18mm. I bought a 23 & 24 and then returned the 24. The flywheel is now off.

Sevanje · Feb 17, 2020

Greetings!
Did you managed to install the starter motor. I am also interested in doing so. On the side note, isn't it when you start having problems like hard starting the culprit is usually the valve spacing and need's to be adjusted?

jbradley · Feb 18, 2020

Sevanje said:
Greetings!
Did you managed to install the starter motor. I am also interested in doing so. On the side note, isn't it when you start having problems like hard starting the culprit is usually the valve spacing and need's to be adjusted?

I have the starter installed, but I haven't gotten around to wiring it, yet. I have to make a short jumper wire to splice into the kill switch, as the wires are too short when just disconnecting the butt splice connectors. Fortunately, it hasn't snowed enough here (SW Iowa) to need the snow thrower, but it has been cold enough that I haven't been ambitious enough to work in the garage. It will be warm enough pretty soon.

The installation is not difficult, and only a bit of a PIA. The kit that I purchased on Amazon had no instructions, so I had to surf a bit to find some. The distributor sent me a copy of the wiring setup, only. The punchout for the starter motor required filing all the way around it, and there are a couple of holes that need to be threaded. The cost is significantly less than adding a new electric start engine, and that in and of itself would probably be a bit of a PIA, too.