I'm moving to a COP setup on my 94 miata, and the toyota COPs I'm using are just a hair larger than the spark plug wire holes (roughly 1 or 2mm).
What's the best way to bore these out and get a consistent diameter without taking it to a machine shop?
Reamer
Shotgun.
Sanding drum in a drill ought to do it. Reamer would be ideal but pricey for a one time use.
2002maniac wrote: Sanding drum in a drill ought to do it. Reamer would be ideal but pricey for a one time use.
Well he did ask for the best way. 
A brake cylinder hone won't work. The hone will wear out faster than the aluminum does. I discovered this while trying to make 54mm bore wheels fit on a Volkswagen.
As someone wwho make accurate holes for a living, "1 or 2mm bigger" is too vague. What's the ID of the bore on a Toyota these are designed to go in? Get that number, measure the ID of the Miata valve cover and you'll know what you need to remove.
A few considerations:
Take the valve cover off and measure the OD of the plug bore - the wall thickness may make this a little difficult.
2mm bigger diameter is a .040" cut if you're boring this - not insubstantial.
a Chinese expanding reamer isn't terribly costly and is the best tool for this if you don't know someone with a milling machine w/ a boring head, which is the the correct tool.
Is this to make more room for the pointy end or the rubber seal at the top? If the latter, make the seal smaller.
Bigger drill
If they don't need to seal anything, then just get a unibit from harbor freight. It will definitely be round, and it will definitely do what you need it to do. And it will definitely be cheap.
If you need it to seal something, DON'T DO THIS.
A Unibit only works on material under about .090 thick. It's ~great~ to punch the hole for a weld-in bung for a wideband O2 sensor, or for an electrician to make holes for conduit fittings in J boxes and load centers.
What you want - really - is a bargain priced expanding reamer. Shars tools sells them cheapest.
motomoron wrote: A Unibit only works on material under about .090 thick.
The 3/8" plate I made my upper spring seats from says they work just fine on thicker stuff, too.
It's still entirely the wrong tool for the task at hand, of course.
Sorry, I assumed he was just taking off the ridge of material on a valve cover where the plug passes through. Did not realize it is the actual BLOCK he is doing. My bad.
As someone wwho make accurate holes for a living, "1 or 2mm bigger" is too vague. What's the ID of the bore on a Toyota these are designed to go in? Get that number, measure the ID of the Miata valve cover and you'll know what you need to remove.
A few considerations:
Take the valve cover off and measure the OD of the plug bore - the wall thickness may make this a little difficult.
2mm bigger diameter is a .040" cut if you're boring this - not insubstantial.
a Chinese expanding reamer isn't terribly costly and is the best tool for this if you don't know someone with a milling machine w/ a boring head, which is the the correct tool.
Is this to make more room for the pointy end or the rubber seal at the top? If the latter, make the seal smaller.
In reply to Knurled:
I'll correct myself to add "Thin stuff unless you want a hole equal to the max diameter of your Unibit"
I'm an outlier since I have literally every number, letter and fractional drill from .010" up to about 1-1/2" plus holes saws, annular cutters, and a milling machine w/ boring heads and bars for the in-betweens.
Problem solved: send it to motomoron and have him do the work!
Looking at a picture of the head, I'm voting on reamer as well.
From the write-ups I have seen about toyota coils on Miatas the only modification necessary is wiring and mounting.
Step drill bit. It's the "correct" way
HiTempguy wrote: Sorry, I assumed he was just taking off the ridge of material on a valve cover where the plug passes through. Did not realize it is the actual BLOCK he is doing. My bad.
It's not the block, just the valve cover.
EvanB wrote: From the write-ups I have seen about toyota coils on Miatas the only modification necessary is wiring and mounting.
My frankenstein miata has a '94 bottom end + VVT head. The holes in the valve cover are ever so slightly smaller.
motomoron wrote: As someone wwho make accurate holes for a living, "1 or 2mm bigger" is too vague. What's the ID of the bore on a Toyota these are designed to go in? Get that number, measure the ID of the Miata valve cover and you'll know what you need to remove. A few considerations: - Take the valve cover off and measure the OD of the plug bore - the wall thickness may make this a little difficult. - 2mm bigger diameter is a .040" cut if you're boring this - not insubstantial. - a Chinese expanding reamer isn't terribly costly and is the best tool for this if you don't know someone with a milling machine w/ a boring head, which is the the correct tool. - Is this to make more room for the pointy end or the rubber seal at the top? If the latter, make the seal smaller.
Alright, I got off my ass and actually measured. I need 1/16" more clearance. It's for the pointy end. Chinese reamer it is!
This is probably obvious, but as you go up in steps (with an adjustable reamer you make a cut, adjust out maybe .005-.010" and cut again 'til you get where you're going) do all 4 holes with each step...
This is an application where WD40 is the right thing - excellent AL cutting fluid.
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