Tiny Power M Twin Build

[racerfrank]

Got off work a bit early today so did some work on the M. Machined both top heads and finished boring and tapping the head bolt holes . A lot of jig boring. I'm starting to see the value of a digital read out.



Until the next time

Frank

[Mike Rometer]

DRO ?

Wouldn't be without mine!!! Wonderfully versatile bit of kit.

[DetroiTug]

Frank, Excellent work! Keep it up and you're going to have a fine running engine.

-Ron

[racerfrank]

I think the next step is to build a crankshaft, but I want some opinions.
At this point I'm not exactly sure what kind of steel the webs were cut from.
On the crankshaft print it says to use C.R.S. (cold rolled) for the shaft but there are a lot of CR steels. I'm thinking 1018 or 1045 since both of those are available with a ground finish and both are easily welded. Good choice?

My plan for machining the webs is to do them in pairs ,or all 4 at once?, boring or drill and ream? to 1.125" (shaft size) then offsetting the 2 inches to bore, drill and ream 1" for the crankpin. After the shaft and crankpin holes are done I plan on putting all 4 webs on a shaft in the lathe and turn then all to a uniform diameter. Out side surfaces that do not clean up in the lathe will be sized in pairs on the mill. All 4 stacked up at once looks like it will be too much.

This is my first crankshaft of any kind all advice or criticism is welcomed.




Frank

[barts]

Note that the outside crankpin holes are bored .984" (25 mm), presumably so that the crankpin will fit through the bearings, so you cannot bore the holes 1" through.

I've not yet built my first crank, but I'd be inclined to do the following:

* rework design to use .984 crankpins all the way around. Personally, I'd make the entire crank like disk 'B' and avoid any possible distortion from welding.
* remove all burrs from the plates.
* tack weld two sets of two plates together at several spots around the perimeter; make sure bottom plate of each pair is smooth and flat.
* mark plates on edge (1 2 3 4) so order is known for later.
For each pair:
* layout the two hole locations on the top plate
* fasten plates securely to lathe face plate, and bore crankshaft hole; re-position and bore crank-pin hole; If you
have 1.125" and 25 mm reamers, bore a few thou under and ream to size; else just bore or lap or perhaps hone.

* make crankpins and shafts, and assemble in order marked in step 4.

- Bart

[racerfrank]

Note that the outside crankpin holes are bored .984" (25 mm), presumably so that the crankpin will fit through the bearings, so you cannot bore the holes 1" through.


Bart--- I should have mentioned that the bearings I purchased have a 1 inch bore instead of the 25 mm. All the other dimensions and load ratings are the same. Instead of a 6205-2rs its a 6205-2rs-1 . I thought an extra $1 for the 2 bearings was well worth the money, being it would save time in making the crankpin and boring/reaming the webs.

Frank

[DetroiTug]

Frank,

Here are some pics of how I completed mine. The crank pins are pressed in the discs and the other end is threaded with a nut on the outside.

All the presswork was done by Pop Arnold as my engine was one of the very first M engines and may be the first twin he ever sold. Castings from 1958. The crank was partly finished, but incomplete - It was two crankshafts for an M single. I bought the whole thing as a box of parts after being in storage for 50 years.

Hope these help.









-Ron

[racerfrank]

Thanks Ron for the photos and thanks Bart for your input .
I think I am going to build according to the plans , with the exception of my crankpins will be 1" instead of the 25mm on the print.

Just need to know if 1018 C.R.S. is a good choice .

Frank

[fredrosse]

For the crank assembly on my engine I used cold rolled mild steel for the webs, and drill rod for the shafts. Tack welded the crank webs together, and line bored in the lathe, with the boring bar rotating in the lathe, and the crank webs mounted on the lathe apron. Line boring of the crank webs all together provides a uniform result. Bored with a single cutting tool set for 0.0005 inch interference fit.

The connecting rod pins are just pressed in, as there is no torsional stress on that assembly. The crank main journals are fitted for torsional stress with "po boy" keys, which is much easier than a typical square keyway, if disassembly will not be required.

I have been using "po boy" keys for years with no problems.

[racerfrank]

Did some crank work the last 2 days. I set the webs up in pairs found my center.
Drilled out to 1 inch.

I then used the boring head and bored it out to finished dimension.

I made my 2 inch offset which gives 4 inch stroke. Drilled and bored.

I repeated that for the second set of webs.
I then put a shaft in the lathe and turned it so the webs would slide on so I could turn the od to size. I center drilled and threaded the end. Slid the webs on and offset the heavy end to control vibration.

I deburred the edges and put them back on the shaft with a turned pin in the crankpin hole it slid in with just a bit of pressure. I need to figure a way to finish the rest, just enough to remove the flame cut kerf marks.



FRank