9/19/2018 8:12 PM
Edited Date/Time: 9/19/2018 9:05 PM
As for a decomp unit:
The little Harley / VTwin 'Pop Out' types have become Very popular. They are nice little units, and some can be very high quality, in Stainless steel.
The smallest ones I've found have a 10mm x 1mm thread. I'd use these in stainless - don't be a cheapskate (hey Jeff C
) and buy cruddy ones, and don't get 'miffed' that they seem to be sold only in pairs......
I don't know the thickness of the 480 head. Neither do I know of the best place to fit one - for accessibility. You'd have to determine that - to what suits you / and the bikes componentry layout.
I would Strongly recommend an adaptor be made, to go into the head, that then the Decomp valve can fit into. To Not have the unit directly in the head, as I doubt that it would be thick enough to go in, whilst still having enough thickness for the valve to open, whilst only having a small decomp hole that - Key issue - would not Ever allow a failed valve, to drop into the combustion chamber. I've seen it happen, and hell, do people want to have their 2t 'drop a valve' and destroy the engine like a 4t can????
An adaptor enables you to say, put in a smaller threaded hole (bugger trying to get in and weld the adaptor in among the fins) into the head - probably a 8mmm x 1.25mm. That then can have around a 2mm / 2.5mm hole in it - you don't need a massive hole / port in the head to get a quite noticeable Decomp effect. The last one I checked for volume around the valve stem and the units 'port', equated to the volume given by a 3mm hole, but you don't need a hole that big. A smaller hole also lessens the tendency for the unit to 'pop closed' when kicking, but they can still be a bit frustrating with that. Better quality units, resist early closing far better than cheap ones.
Then, on the Decomp unit side, you have the 10mm x 1mm threaded section, machined with enough depth to that pocket to give around 2mm clearance when the valve is 'open'. The gas can easily flow around the valve. I've generally made the adaptors out of stainless steel, but also done them in aluminium.
The adaptor set up also brings out the Decomp to a more reachable point - Fins, boys, on a 480 Air Cooler - and, of course, you probably will have to machine a bit of 'finning' away. Though, a bit of careful thinking about the Decomp landing may mean No Fin trimming may be needed - again, I'm not that familiar with the 480 / 500 Air Cooled heads.
As I said, I can't offer any specific area to place it - it's down to the points I mentioned at the start. But I Do Not favour placing a decomp hole / port in the Squish Zone , though I've done it when necessary. In the 'bath' of the head, and , generally following the 'arc' of the combustion chamber is best.
To those that think an 'in head' Decomp port will lower working / firing compression - no, it's not an issue. It only comes into play when the valve is open. I've fitted Decomps for years, and have Never had anyone say their bike did not run the same, after it.
All Long winded, I know, but I try to impart information as well as I can.
