Now that is the second 1505 crank I've seen this week broken in exactly the same place!!!

Are they a problem?

I believe the big journal cranks are more break prone than the small journal ones, as they cannot flex as much.
Some engine builders grind the big journal crankpins down to 1.625" (whether stroked at the same time or not), they reckon this adds reliability.

#1 cause of general breakage though seems to be too small a corner radius, or a poorly formed radius (with a step in it) after crank regrind.
Of course, this comment applies to other A series cranks too, just more so on long stroke 997, 1098 and 1275 ones..

[edit] typo

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

There certainly could be merit in that, however (and it it difficult to say) maybe not necessarily from allowing the crank to flex more but by grinding the pins smaller lightens it and in doing so, moves the harmonics of the crank to a higher rev range.




Yes. When are these $%^&en so called crank grinders gunna learn how to, well, grind a crank....

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You can grind a 2 foot radius and it is not going to change the torsional vibrations which are causing the crank to crack.

1 you grind the pins back to 1.625 and what rods do you fit, not the big heavy ones that were fitted originally. The extra weight of those rods effects the "balance " of the crank

2 The factors contributing to torsional vibration are peak pressure of the cylinder, the acceleration of the crank, and mainly the stroke of the crank. This determins the rate of acceleration of the piston/rod in relation to the linear/ reciprical motion of the crank. Bigger the bang in the cylinder bigger the force.

I have seen big journal cranks that have been radiused and still crack/break. Look at all the cranks that crack and break and I think you will find that they all have a common factor. Most of the 1100 cranks that I have seen break do it in the same place[unless rod goes first then takes the crank out] 998's are in a different place.

Now all crank shafts have critical points where they are subjected to a maximum force.
Crankshaft flex is a minor factor and think about how a mini engine is different to most other engines and what occurs to the crank if it does flex and why it does.
The bottom line is it is not the fault of the crank but the other contributing factors acting on the crank.

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

You fit Cooper S/small journal 1100S rods (lightened a bit), or aftermarket rods of your choice...
If I have to use the original paperweight ones with a big journal crank, I mill the damn paperweight off, then balance them.
1275 cranks generally have very poor counterweighting. Wedging these cranks helps lighten them, as well as improving the balance factor. So then the main bearings don't wear more one side of the journal than the other.

[edit] for clarity

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yaaaaarrrgghhh

gotta love it when Dave and the Doc get down and dirty

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"Show me the Mini!"

Exactly my point, when you go back to 1.625 then you use a lighter con-rod,but my question is "why do they crack and then break and why do 1275 cranks, 1100 cranks break in the same spot[although there may be some that don't] and 998 cranks break in a different place.

From what I have seen over the years the two places are in the front[timing end] of the center main and the 998 seem to go thru the web between 1 & 2 big ends.

As for balancing is it better to remove metal or add metal. What requires more energy to move the crank or the flywheel/clutch/ back plate.

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

997 cranks break too, the one I saw went through the front web.
Same as my old 850 did (850s have thin webs).
As for 998 cranks, I've never seen one break- maybe they do but IMO they are more bulletproof than the rest, if you are gonna rev em.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I have a couple of cracked big journal cranks that I pulled out of engines before they broke and the cracks start on the rotational side and timing end of the web. One is further advanced than the other but the cracks have not exited thru the journal face yet.

I also have had 1100 cranks the same and I have broken only 1 998 cranks so far. That one went thru the web between 1&2 adjacent to No2 big end.
The balance of the crank is the important factor but when you look at the position of the cracks then there is a overiding force that is effecting the crank. Why do they not break at the rear main or the front main bearing.

When you look at the firing order of the engine and the 2 major forces on the crank,IE peak pressure and compression, 3&4 are adjacent and the crank end 1&2 is not under load and this end of the crank acts as the damper.

with any force if you support the componet then the magnitude of the force is absorbed. As I said some time ago ,the biggest sudden change of force on a mini engine is the operation of the clutch due to 2 factors.
1 the back plate going out of balance due to the metal strips flexing
2 the change in load[crank twist] from the clutch engaging or dis-engaging.
during clutch operation the crank is pushed against the center main [thrust] on the fly wheel side and the force is absorbed into the main bearing/block but the other end of the crank is free and "whips". This also might also be the reason why the center main caps could fail. Hence people stick lumps of metal on them to stop a cap failure.

On most other engines the clutch/pressure plate is located precisely onto the flywheel and there are no other bits of metal floating around to cause a "out of balance force"

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

The cracked small j crank I had was cracked along the radius of the journal on that exact same spot

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Lillee - 1969 Morris Mini K