Bolt circle diameter

[dat dude]

Does anybody know what the bolt circle diameter is for the cylinder head studs on a CR250? The thread in the cylinder stripped out and I'm repairing all 6 threads using a helicoil. I need the number so that I can drill the threads out using a CNC mill. I can take a measurement, but wanted to know what the exact value is supposed to be. Any help appreciated...gracias!!

 

[Ol'89r]

dude.

If my memory serves me correctly, :eek: :laugh: the studs on your head are 8X1.25mm. You will need to use a 21/64 drill for a HeliCoil. The cylinder base studs are 10X1.25mm and they use a 13/32 drill.

Just my $ .02

Ol'89r

 

[dat dude]

You're right, but I need the diameter that all 6 studs sit on (an imaginary circle that goes thru the centerline of each stud)

 

[Ol'89r]

Alrighty then. How about 3.850" outside to outside. :thumb:

Jeeze! I gotta start paying more attention. :scream:

 

[SFO]

3.85"?
If you have a 6 hole equal spaced BC then I would just spot out the BC #'s by using a caliper on the clearance holes in the head. Either using the inner edges or the outer edges of the opposing holes and subtract the dia of one of the holes. Convert this size to metric, because these dims usualy are nominal in whole mm sizes. It is just a double check though, you already have a mating part to use as a gauge.
Hope thids helps

 

[DEANSFASTWAY]

Maybe clamp the head down on the cyl correctly and use the head bolt holes as a drill guide. or measure on cyl head or head gasket. Some years Honda studs are stepped and the base is a 10mm stud and the nut or exposed portion is 8mms. The stepped studs are better.

 

[dat dude]

Thanks for the suggestions. The only reason I asked, was that it seemed like they were not equally spaced tapped holes. When I tried to put the cyclinder head on in a position other than the way its supposed to fit, it would not fit over the studs. Weird, but it looks like the studs have been replaced before, and somebody had to repair the threads the same way I did, and didn't do too hot of a job. Thanks again for the help. Another quick question...what are all of you using to torque down the 4 bolts that fasten the cylinder down to the crankcase. A box end wrench fits fine, but a socket doesn't fit so getting an accurate torque value is difficult. Friend of mine has a 14mm open end wrench that is cut about half way down the handle, and has a socket welded to the cut end. This works, but you need to lower the torque value on the wrench to counter the extra leverage of the wrench. What is everyone else using???

 

[dirt bike dave]

The holes in the head and the head studs on the cylinder are not equally spaced on the CR250. The head will only slide over the studs if you put it on the right way.

I'll probably get flamed for this, but as far as torquing the cylinder to the crank case, I use my finely calibrated right wrist and a box end wrench. No problems so far. Try practicing with both your torque wrench and the box end wrench on a bolt that is easy to access, so you know what 29 lb. ft. feels like. As with the head, criss-cross pattern in a few steps before you get to the desired lb. ft.

 

[Ol'89r]

Originally posted by dat dude
A box end wrench fits fine, but a socket doesn't fit so getting an accurate torque value is difficult. Friend of mine has a 14mm open end wrench that is cut about half way down the handle, and has a socket welded to the cut end. This works, but you need to lower the torque value on the wrench to counter the extra leverage of the wrench. What is everyone else using???

dude.

The wrench that your friend has will work just fine, but when you use it, it has to be set at a 90 degree angle to your torque wrench. With it set that way, you can use the same values.

I personally use dirt bike daves method and have never had any problems.

Ok, now we are up to about $ .04 :confused:

 

[bclapham]

i love the way you guys use a standard drill bit to drill a metric hole! :confused: i get loads of funny looks in the hardware store since i walk around with my calipers figuring everything back into metric

i also "use the force" on the bottom studs, but i kind of get worried describing it as the "finely calibrated right wrist method" :eek: tell me, what should i use to calibrate my right wrist and what sort of precision can i expect from my left wrist???? :p

 

[490Dave]

I also have a box end wrench welded to an old socket to use on those nuts, but i use the torque wrench, just cuzz i have one i guess.

I think it's the right hand that installs and the left that removes, they dont need to be calibrated or synchronized.

 

[SFO]

If you have a CMM you could spot the holes out and get your X~Y coordinates that way.
Or, use your DRO and actually dial in each hole on your head with an indicator.
Either way you can generate the needed coordinates with what you have.