Tips for Crankcase Machining From MAXNOVO MACHINE Team

MAXNOVO MACHINE Tips : Each month I receive nice compliments from readers, often followed by a technical question. The time required to generate the required response goes un-noticed in the warm glow of the afore-mentioned compliments, so everybody wins — I think.   

This past month, a builder in Norway sent the picture here of a case for a 2.5CC design he had cast—from a Peugeot cylinder head, no less—and machined, noting that he believed the crankshaft axis and cylinder bore were not perfectly at right angles. He was concerned, and was wondering just how concerned he should be. That, as they say, this is a very good question.  

Our standard procedure for machining the bore is this: first the crankshaft bore is machined, and the rear the case faced off at the same time to assure it will be as nearly at right-angles to the shaft bore as your lathe will permit. If the case is to be screw-cut internally for a threaded backplate, this is done after the rear has been machined in case the thread-cutting shifts the setup minutely.

The photo here, of the "ML Midge" case, shows how the rear face is then used in conjunction with a small angle-plate mounted to a lathe face-plate to drill and machine the cylinder bore. The accuracy of the cylinder bore will be governed by the accuracy of the angle-plate, how clean your spindle nose and registers were when the face-plate was screwed on, and how clean the area under the case is. The piece of paper in the photo is an old business-card used to protect the case finish, and to increase friction of the set-up. Card stock generally has a very consistent thickness.

Back to the original question. I did a little quick, theoretical math based on a "square" (equal bore and stroke) engine of 0.6" (making it just over 2.5CC) and arbitrarily deciding that any mis-alignment that caused the piston to go 0.002" off at TDC might be bad. It turns out that this corresponds to an error of just 0.1 seconds of arc off a perfect right-angle! Now I don't care how good you are, working consistently to this sort of tolerance is insane.

Now, it is time to call on real experts. Mr David Owen knows engine design and industrial manufacturing. His reply was as follows : It is an interesting question with, as you say, frightening mathematical conclusions. The diametral crankpin clearance alone on say a 2.5cc engine will be in the region of .01 to .02mm for a hydrodynamic bearing of say 6mm dia. Extrapolate this to a bearing length of say 5mm and a conrod length of say 25mm and it will be obvious that no force exists to displace the piston at TDC until there is an angular error of the major axes in the range of 4-7 minutes of arc. Add to this some little end clearance plus the effect of a tapered bore and we have some latitude.  

However, the consequence of cumulative misalignment will be the tendency for the conrod to run first to one restraint, then to the other, with a consequent reduction in efficiency and an increase in likely wear rates. Given that the crankshaft bends, some builders deliberately introduce an error in the alignment of axes. Others grind the crankpin to a reverse taper. I think both these courses are unnecessary in the simpler type of engines most of us build.

Fortunately, the practicality of all this is not so daunting. If your friend machines the crankshaft axis and some vertical face, generally the backplate sealing surface, at the same setting, he will have a suitable reference to machine the cylinder axis at the optimum 90 degrees. An angle plate, if used should be checked for truth when mounted on the lathe or under the mill. Any 90 degree fixture on the lathe is easily trued in-situ with a facing cut, given alignment far better than my 4-7' range.

As a result, there you have it. 4--7 minutes is certainly within the scope of home-shop equipment, so we can all stop worrying and get back to engine making. Just be sure things are clean, follow the machining sequence given above, and you will certainly be in the ball-park.   

Do you need High Precision Lathe and CNC Machines ?  To get more information, always welcome you to contact our MAXNOVO MACHINE Team ( www.maxnovocnc.com ) and our experts are happy to support you.