Yz490 decompression valve

[Big Scotty]

Gday
I'm about to fit a decompression valve in the head of my 490,my question is do I drill and tap the 10mm hole straight through the head or do I make a stepped hole with a smaller diameter hole on the combustion side,the decompression valve is only about half the length of the spark plug thread which is the same thickness of the head,any ideas thanks
Cheers scotty

[Momus]

I've done few and used a 2.5 mm hole into the combustion chamber and kept the dead space below the valve head to a minimu- so stepped hole.

[OverTheHill]

On my yz465g in 1981 after it was a year old & wanted to use it in enduros for a change, i drilled a hole exactly like AG175's or RD400's have to soften the comp for kick starting. Might've made it slightly larger than i should've but worked a treat. Did sound fluffy at idle but still rip snorted above that just the same. Hole is open all the time as you've probably gathered on the models mentioned. Was worried drilling it but never broke through into open air & was pretty happy when it arrived in the exhaust port outlet. Plan was [if i didn't like it] to tap a grub screw in at the bottom end to block it off--but sold it & bought an "82 490J lol--but i won't go into that story with crap rear suspension. Back to the hole--i can't exactly recall the position but think just below halfway between top of port & top of cylinder, maybe a 1/3 up from port. Wasn't easy to start the drilling by memory but possibly ground a mark in cylinder wall. Could damn near sit on the seat to start it which helped when stuck in difficult situations in an enduro & stalled it trying to trials ride around an obstacle. Bloody silly idea in the first place using it as an enduro bike. IT250 would've been 'the go'.