Flywheel Key Dimensions

I have a C&P 8X12 that’s almost ready except for a missing flywheel key. I know I need a tapered key, and that I can get one from McMcaster-Carr, but can some kind soul tell me what are the key’s dimensions - fat end versus thin end?

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Basic rule of thumb from way back. C & P from a long time ago will probably be dimensions all round in Imperial measurements. Probably making the slot(s) shaft & flywheel in approx equal division,i.e.(again) just approx. 1/4” in both (width) and total depth for key 1/4” & 1/8” x 2.
Possibly construct simple test gauge from piece of lead (pica/12 point) to ascertain exact size >height/width< of keyway, virtually always parallel to accomodate either straight key with grub-screw or tapered key
As taper keys are generally regarded as sacrificial, for obvious reason, possibly source 2 or 3 mild steel, calculated to fit gently at about 50% of total usable length and file down lengthwise, progressively to give overall contact internally, using, small dab of printing ink as *Engineers Blue*.
System well documented, W W W. and /or B.P.

The eventual rake along the length, may be ascertained from the Web as applied to Taper Pin dimensions, roughly comparable for starters.!

Even a Key or Keys sourced off the shelf would probably benefit from precision fitting.?

N.B. always good forward planning, during and prior to eventual fitting, as Taper-Keys normally have an extractor lug at exactly 90 degrees, normally with careful use O.K. but even removing with tiny Slide hammer, Extractors or Wedges always end up with negative bias, and slip off, hence 2 or 3% inward positive bias with file (on the lug) usually pays dividends.

Regards to Sarah Palin or Lisa Kelly if your ever in Wasilla
Good Luck. Hank.


Old series or new series? Don’t know if it makes a difference but there are other differences between the two so I wouldn’t assume that they are identical.

Hi Greg,

I ordered 2 of those for my 7x11 (same groove measurements as my 8x12), filed them down .02” and they fit perfectly. I could have gone with the 1.5” key and probably avoided the filing, but this gives more surface contact.


Thanks, Brad. Is it possible to buy a key already filed to the right dimensions? I haven’t found anything at McMaster like that.

Hi Greg,

I don’t know of any keys that will be already filed to these dimensions. I’ve almost never seen a key that fits any old machine without some grinding. I just used a hand file and it took only 20-30 minutes for both of the keys.


it is a standard taper and the key you get from Mc Master will be close. final fitment is up to you.
be sure though, to clean up the sides of the groove in the crankshaft and flywheel separately. the key should slide smoothly, but as little sideplay as possible.
if you contact me, i can send you a link to a video of the whole process. it is too involved to explain here. seeing is better.

That a relief, Brad. I don’t live near any machine shops but I do have some blank keys from McMaster, a vise, and a file. So I’ll give it a go.


Thanks, Hank. Sarah and I aren’t on speaking terms.