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metric thread pitch..... compared to inch thread pitch...

m8 closest outside diameter in inch is .314" which is 5/16
m6 closest outside diameter in inch is .236" so use a 1/4 x 20 threads per inch or 1/4 28 tpi

so u have a few options dude... drill and tap it bigger... and use a heli coil.. or drill and tap it so a 1/4 inch bolt will fit...

so.. to find drill sizes for taps this is how u do it..
1.. u have a tap drill chart.. and find the coresponding drill for the tap u use
2.. use a formula..
metric... use this one.. diameter of tap subtract the pitch = drill size
tap M6 * 1.00 example M6 - 1 = 5mm drill diameter for the m6 tap
if u dont have a metric drill... u can use an inch drill... just convert the millimeter (mm) to inch measurement.. like this
mm times 0.03937= inch measurment
example... 5mm * .03937 = .19685" ... so u use a 0.197 or a 0.194 inch drill for that metric tap...
.03937 is the magic number used to convert MM to inch

for a inch tap such as 1/4 * 20 TPI use this formula
tap size(major diameter) subtract the pitch = your drill size (minor diameter)
so example
1/4 - 1/20 = drill size
.250 - .050 = .200 drill size
now .200 is an odd size drill to find.. so what u do is go buy a 3/16 (.187) drill and use that to drill ur hole... ( every hole drilled will drill oversized by .010 so your .187 will infact be .197 by the end of the drilling.. so its good for your 1/4 tap

now.. after u drill the hole.... let the aluminum cool a bit... (water on this to help is ok) if u dont let it cool and tap right after... the tap may gall up and tear ur threads...
always cut metal cool never hot or warm...

get a small square and use it to square up the tap with the hole.. also do this with the drill before drilling

now dude... here is what u do..... me personally im a machinist.. and i do this crap everyday... so the closest bolt to fit in that stripped hold is a 1/4 * 20 bolt..
ur block is aluminum so.. use coarse threads...
1/4 * 20 threads per inch (TPI) needs a drill size of .200

now u just fixed ur bike for the cost of the tap and drill bit... and learned how to do it in the process... so you will never have to pay anyone to do it every again...LOL bad for me cause i rely on people messin up there crap so i can fix it...lol

taps come in a set of 3.. a starter tap... a finishing tap is all u need... u can tell by the taper at the beginning of the tap.... large taper goin up about 6 or 7 threads is the starter.. small taper going up 1 or 1 .5 threads is the finishing tap...
u use the finishing tap to tap blind holes and bottoming out the threads for ur bolt..

pm me if u need any more advice
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