Just a question of curiosity
If I had a crank with a big end of 50mm and a rod bigend of 44mm and offset ground my crank to 44mm big ends.
So that's a 6mm offset grind. How much stroke do I increase??
My thinking is it will change the bigend centreline 3mm so I gain 3mm at the top stroke and 3mm at the bottom stroke, so a gain off 6mm total stroke?

Anyone?

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When the sign says "don't feed the bears", man you better not feed the bears.

You are on the right track - yes, you change the centreline of the rod big end journal by 3 mm (radius changes from 50/2 = 25 to 44/2 = 22 so difference is 3 mm) so your stroke change is 3 mm, not 6mm. Assuming it is ground the correct "offset" direction, ie. making the new centreline move away from the crank centreline, the engine stroke is increased because you are lengthening the cranks "lever arm"; of course, you can destroke by offset grinding the "other" way and shorten the lever arm accordingly.

But if I'm raising the bigend centre line by 3mm.
Wouldn't I then raise tdc by 3mm and lower BCD by 3mm adding a stroke increase of 6mm total?

When I put my z24 crank in my l20b it raised tdc by 5mm and lowered BCD by 5mm with a 10mm total stroke increase.

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When the sign says "don't feed the bears", man you better not feed the bears.

I think you (probably me?) are maybe confusing the stroke (crank "lever arm”) definition I mentioned with displacement (swept volume) that is generated by the increased stroke created by offset grinding the crank. You are correct in that the TDC and BDC points are now higher and lower respectively, by the amount of the new “lever arm” increase. So, yes, it is true that the additional "stroke" increases the volume displaced per revolution equivalent to 2 x the new “lever arm” dimension.

Sorry for the confusion...hope this clarifies this?

Stroke is double the distance from crank centreline to big end centreline, so if you gain 3mm extra from crank to big end you will indeed gain 6mm stroke.

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady