Max Crank Grind

[jonclancy]

Hi Folks,

Google hasn't really helped, but what is the max grind on a 1275 crank, please? MED list 0.010 and 0.020 only.

I found the following in my workshop manual:

1275 cc No more than >010 without heat treatment

Mine is already 0.020 under. 1098 cranks can be ground to 0.040 under, according to my manual.

Many thanks!

Jon

[cs3tcr]

The EN40B 1275 cranks cant go past 010 due to nitriding. But the later cranks can be ground smaller, Moss Europe lists 060 under, but if it were me I would seek another crank if you have to go past 030.

[jonclancy]

Thanks! This makes sense - my manual covers early Sprite IV and Midget III. I'll go and check my crank for the material...

[jonclancy]

OK, my crank is EN16 = tuftrided.

The good news (or better at least) is that I have collected my spare block, crank and pistons from the workshop. It's EN16, too. But the journals are in much better condition. I hope to find some shells from it on a box to get a clue as to the undersize. Anyway, it'll all be going up the road in the next few days. I'll post an update when I know what's what.

[erictharg]

You can go below -010", but will need to have it re-nitrided or tuftrided. All comes down to bearing availability and cost. Not mega bucks either way.

[Westfield 129]

Mic the crank and you will know its condition. If the journals are not torn up, it is likely that you wont have to grind the crank. You can just polish the journals, using the standard bearings.

[jonclancy]

Update:

Original crank will need a grind as it's a few thou worn (with surface marking). Not a problem, and it'll be a 030/030 item when done. Hardening as desired/required.

The good news is that my spare crank is std/std and pretty much bob-on. It'll just have a polish and std shells. Pistons etc look good. Flywheel will get lightened and have a new ring-gear fitted. The whole lot will be balanced.