AJP cranks

[Jhonno]

This old chestnut.. Reading though many old threads currently. I have a small journal crank in my AJP which replaced a snapped small journal one in 2008/9.. Is there a version of the small journal crank that doesn't fail? Will the stronger 4.5 large journal crank work with my 4.2 rods and pistons?

I found a thread saying you can get it tested to see if it is a good one or not? A valid option?

[Steve_T]

The 4.2 rods likely won't fit a 4.5 crank because the stroke is different - you would need to be lucky that the difference in pin height accounted for the difference in stroke. You could test your 4.2 crank for cracks, but even if it is not cracked I don't know how you could test its remaining life.

[Jhonno]

Steve_T said:

The 4.2 rods likely won't fit a 4.5 crank because the stroke is different - you would need to be lucky that the difference in pin height accounted for the difference in stroke. You could test your 4.2 crank for cracks, but even if it is not cracked I don't know how you could test its remaining life.

Stroke is the same.. 4.5 comes from a bigger bore.

[Steve_T]

With the stroke the same, are the pin heights in the 4.2 and 4.5 pistons the same, since if they are then the rod lengths are the same as a consequence. You've got the big end bearing diameters to consider and also what the weight differences between the 4.2 and 4.5 rods and pistons. You don't want to have to account for a big difference when you balance the crank. Suspect easiest to run it past an AJP engine builder.

[Mr Cerbera]

Jhonno said:

Steve_T said:

The 4.2 rods likely won't fit a 4.5 crank because the stroke is different - you would need to be lucky that the difference in pin height accounted for the difference in stroke. You could test your 4.2 crank for cracks, but even if it is not cracked I don't know how you could test its remaining life.

Stroke is the same.. 4.5 comes from a bigger bore.

I've read "the decision was made to reduce the bore from 94mm to 91mm but increase the stroke from 75.3mm to 86mm, giving a new 4.5 litre capacity"

Just sayin....

[Jhonno]

The 4.2 is 88mm bore x 86mm stroke..

4.5 is 91mm bore x 86 stroke.

[Mr Cerbera]

Jhonno said:

The 4.2 is 88mm bore x 86mm stroke..

4.5 is 91mm bore x 86 stroke.

I bow to your superior knowledge, Sir
(and Thanks for posting )