Crank journal sizes

[nick rogers]

I recently sold a 1071 crank which I had measured at .060" mains and .040" big ends. The customer returned it as his local machinists found it to be .001" (one thou) under size on the big ends and .0012" (1.2 thou) on the mains. This got me thinking about : (a) Machining tolerances. What do crank grinders consider reasonable. (b) Oil film thickness. How thick is that ? and ( c) coefficients of expansion of EN40 steel compared with cast iron. Do all engine components expand at the same rate ? I assume the crank will be at about 120 degs if the oil is at that temp.
I would welcome opinions.

[rich@minispares.com]

The customer returned it as his local machinists found it to be .001" (one thou) under size on the big ends and .0012" (1.2 thou) on the mains.

in the AKD4935 the running clearances for the bearings is listed as 0.001 - 0.0027, so I would say that your crank is well within spec.

im sure simon will know for sure

[251 ENG]

Big end size is 1.6254" - 1.6249" .

The problem comes with the main size as BMC , Vandervell and Glacier have all listed different sizes over the years .

I try to work on 2.0008" - 2.0012"

The problem is that the bearing housings in many blocks are all over the place now .

You need to aim for 0.002" bearing clearance on the mains

[Spider]

What are the measured sizes of the crank journals? and really, it needs to be assembled and checked with Plasigauge before determining if it's out of spec.

Big End clearance is 0.001 to 0.0025", however best aim in the region of 0.0015 to 0.0022".

[nick rogers]

The crank in question was not freshly ground, it was one I had removed from a block. I had assumed the journals at 1.585" big ends and 1.94" mains. The shells had indicated .040 under and .060" . I checked it visually and gave it the lightest of polishes with 800 grit emery.
I think your comments support my theory that the machine shop he showed it to were being a little over-cautious. The crank is now back with me and I may use it in a 1071 of my own and sell my fresh .020/.030 one. Thanks to all.

[goff]

Nick
why don't you take it yourself to a couple of Crank grinders and see what they say !!!!!!!!

Goff

[leonrjohnson]

I recently sold a 1071 crank which I had measured at .060" mains and .040" big ends. The customer returned it as his local machinists found it to be .001" (one thou) under size on the big ends and .0012" (1.2 thou) on the mains. This got me thinking about : (a) Machining tolerances. What do crank grinders consider reasonable. (b) Oil film thickness. How thick is that ? and ( c) coefficients of expansion of EN40 steel compared with cast iron. Do all engine components expand at the same rate ? I assume the crank will be at about 120 degs if the oil is at that temp.
I would welcome opinions.

0.060 and 0.040 is a lot to grind off a crank! lucky to find bearings to suit. I wouldnt use a crank that had been ground this much.

[Vegard]

I recently sold a 1071 crank which I had measured at .060" mains and .040" big ends. The customer returned it as his local machinists found it to be .001" (one thou) under size on the big ends and .0012" (1.2 thou) on the mains. This got me thinking about : (a) Machining tolerances. What do crank grinders consider reasonable. (b) Oil film thickness. How thick is that ? and ( c) coefficients of expansion of EN40 steel compared with cast iron. Do all engine components expand at the same rate ? I assume the crank will be at about 120 degs if the oil is at that temp.
I would welcome opinions.

0.060 and 0.040 is a lot to grind off a crank! lucky to find bearings to suit. I wouldnt use a crank that had been ground this much.

I would. Still much bigger journals than a 998.

[251 ENG]

Nothing wrong with undersize cranks if they are crack free and ground with correct fillet radi

And you have the bearings for them .

[Spider]

Nothing wrong with undersize cranks if they are crack free and ground with correct fillet radi

And a good harmonic damper!! Very important.

Many of the racing guys used to grind their cranks down to the smallest size for which they could get bearings for, right off the bat. Small dia = less drag, though it's really getting to academic levels!! Not something I'd do unless needed, but at -0.060" it is no reason to reject the crank.

[251 ENG]

When you nitride EN 40B cranks they will grow slightly , normally the pulley diameters will be 0.001" too big and will need polishing down .

Journals don,t grow that much and you take the chance that the crank will not be straight after heat treatment.