How do you size a spinner for a gas engine? I just purchased my first gas 26cc engine and need a spinner. I also see spinner bolts? These are for sale separate from the spinner. Do you need a special bolt for gas spinners? The cal has a 3” flat service so would it be a 3” spinner? Also do you need to cut them for the prop size?

Spinner size is directly related to the aircraft not the engine. It sounds like a 3" is your ticket.

As far as spinner bolts / adapters that is directly related to the engine. Some engines have the prop bolt tapped for an 8/32 bolt or something.. some need a prop adapter. If the prop bolt is tapped put a bolt in there to see what it is tapped as and get one long enough for your spinner. Usually use black alloy bolts myself.

Most tapped bolts are 10-32 threaded, to stay in line with Tru-Turn and the followers making machined aluminum spinners and the spinner bolts they use. For especially long spinners (distance from tip to the threaded prop bolt) Tru-Turn has long custom bolts for this purpose, but most of the time you can get lucky at your local hardware store, or custom fastener shop, such as Fastenal. Your spinner may come with a backplate that simply has an approximate 10mm hole, and some adapters to fit different crankshaft sizes. In this case there are spinner adapter kits for various crankshafts. All of this will depend on the brand, size and type of spinner you end up with, and of course, whether your engine has a threaded crankshaft, or a threaded prop bolt that attaches to a prop hub on the engine. What 26cc engine are you talking about here?

SPE26cc Heres the link http://www.greenmodelusa.com/SPE_GasEngine.html

Do you have to trim the spinner to fit the prop? 18x6?

Depends on what spinner you but. The upper end spinners need to be cut, the everyday type spinners are already cut. You may have to trim a little to make things fit. Just like a large glow engine with a big prop. There's no difference between the two types of engines when it comes to spinners. No worries.

Thanks for the input.