A lot depends on whether you are using stock or aftermarket valves (like KPM) and cast iron or bronze guides. You could use your 5/16" HSS reamer on cast iron guides, but it would probably make the fit too tight for stock valves. If it's never been used before, you could also use it on bronze guides, and it might work out ok with stock valves, and it might not. Stock stems can vary from .3105" to .3115", and the factory spec to the guide ID is .3135" to .3145", so for cast iron guides, you'd probably want to use a .313" or .314" reamer, depending on the size of your valve stems . For bronze guides you need to run tighter clearances. I use HSS reamers reamers from Goodson that are purpose made for valve guides. They are about $50 each. Valve guide reamers are normally spiral, with a smaller pilot at the end, and a slight taper to get the cut started. They should be dedicated to bronze guides and never used for anything else. Once you use them on cast iron guides, they will not cut well in bronze. For Commando heads with bronze guides I use .311" and .312" reamers, depending on the valve stem dimensions, and if I need a little more clearance I use a valve guide hone. I normally run .0005" - .001" on intakes and .0015" - .0020" on exhausts with bronze guides. More info on the reamers from Goodson at
www.goodson.com
Goodson will tell you not to use the HSS reamers on bronze guides, only carbide reamers, and that's probably reasonable if you're doing a lot of guides. But the HSS ones have worked well for me on bronze guides, and they are a fourth the cost of the carbide ones. A good cutting fluid helps a lot.
There are other suppliers that offer similar reamers. I have a stack of old HSS valve guide reamers made years ago by Sioux, and they still work fine for old iron automotive heads.
If you're going with KPM valves and bronze guides, then the simple solution is to use the KPM reamer.
And yes, the simple answer is to send the head to a machine shop that has experience with Norton heads, but I understand the desire to do your own work. If it goes well, you've learned something, and if it doesn't, then you can send the head off to get it done right.
Hope this helps.
Ken
