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Crank Pro/Cons
https://www.ausmini.com/forums/viewtopic.php?f=2&t=69498		 Page 1 of 1
Author:  Hanra [ Wed Mar 16, 2011 8:41 pm ]
Post subject:  Crank Pro/Cons
What are the pros and cons in regards to using a 1275 (Non S) Small Journal Crank compared with a 1275 (Non S) Large Journal Crank?
Author:  drmini in aust [ Wed Mar 16, 2011 8:52 pm ]
Post subject: 
I've seen more cracked big journal ones than small journal.
Also the rods on big journal A series weigh a ton.
IMO the best use for a big journal one (uncracked) is to stroke it, wedge it, balance it then fit S rods, but I'm biased... :P
Author:  Hanra [ Wed Mar 16, 2011 8:57 pm ]
Post subject: 
And Supercharge it?
Author:  drmini in aust [ Wed Mar 16, 2011 9:03 pm ]
Post subject: 
Hanra wrote:
And Supercharge it?

If it floats your boat, why not. 8)
Author:  GT mowog [ Wed Mar 16, 2011 9:14 pm ]
Post subject: 
I've seen about as many small jounal ones cracked as big and in all fairness, there are more big journal ones about. The late A+ rolled radius cranks are about the 'strongest' ones of the lot.

The 1100S style rod is heavy (can be lightened if you can be bothered), as the Doc has already pointed out, but the A+ Rods are so much lighter, quite a good rod in fact.
Author:  michaelb [ Wed Mar 16, 2011 9:35 pm ]
Post subject: 
I have a 1275LS crank and after some advise I then aquired a set of A+ rods which do not have the big counter weights of the original 1275LS rods.
The crank has now been wedged and balanced.
One day it will be assembled and have a Supercharger assisting it to spin. :wink:

Image
Author:  Hanra [ Thu Mar 17, 2011 1:04 pm ]
Post subject: 
What sort of blower will you run?
Author:  michaelb [ Thu Mar 17, 2011 1:17 pm ]
Post subject: 
Quote:
What sort of blower will you run?

Eaton M45 - Vmax Scart kit from the UK.
Author:  david rosenthal [ Thu Mar 17, 2011 2:30 pm ]
Post subject: 
during the discussion on the other crank thread one of GR's comments that the amount of overlap between the big end and main journals made the crank stronger to withstand cracking. He quoted various other engine cranks etc.
OK so to the various mini cranks and as we know a large journal crank "sometimes" will crack more than a small journal crank.
Now most of the cranks will crack/break thru the web between the center main and No.2 big end, so as GR said if the overlap between the two journal diameters is greater ,the risk of breaking is less.

So here are the dimensions of various mini cranks and the amount of overlap between the two journal sizes.
Image
Now going by these dimensions a big journal crank should be stronger. The overlap in a 998 crank is less than a 1275 so they should be weaker and more prone to breaking. A 1100 crank ,which are very prone to breaking has a overlap of .039", and if you stroke a 1275 big journal crank the stroke ends up .125" more[shift the center line by .062"] and the overlap ends up at .1525"

The other dimensional difference between a 1275 and a 998 crank is the distance [thickness] between the thrust face at the center main and the face of the big end. This cross section area is where the crank breaks, and a 1275 is less than a 998, but only by .020". The web is wider in the taper from one side to the other and would give a greater/stronger section.
The other thing different between the cranks is the cross sectional dimension of the webs between the 2 big ends.
Image

So if "stiffness" is a issue with the B/J cranks breaking, then a B/J crank should be the strongest. IE the most overlap and the greatest supported area.

The" solution" to the problem is to grind/roll a big radius from the journal face to the thrust face in the assumption that this is going to stop a crank shaft from cracking/breaking thru the web. Now if the crack intersects between the center main and the big end then why is the big end not also radiused.
The other point is how/where/why does the crack originate. A radius will relieve a stress point but the idea of a radius assumes that the start of the crack is originating at the brg face and exiting thru the opposite ends of the web.
If this is the case then the ONLY way this is going to happen, is for the crack to be bent longitudinally[maximum point of stress] and this is not going to happen as the mains stop that.
Author:  TK [ Thu Mar 17, 2011 7:28 pm ]
Post subject: 
All of the small journal cranks I have owned (plus 10) have been cross drilled. Factory?
As Kev said with a small journal crank you can use the lighter stronger rods. :)
Author:  drmini in aust [ Thu Mar 17, 2011 7:36 pm ]
Post subject: 
Many/most `small J' 1275 cranks were cross drilled, but not all, including early Cooper S EN40B ones.
I have 1x EN16 crank in the collection which isn't cross drilled.
Author:  Mike_Byron [ Thu Mar 17, 2011 7:39 pm ]
Post subject: 
GR has also said that the biggest cause of cracking cranks and centre main bearings is tossing back a gear at revs and dumping the clutch.

I know that they do crack regardless though but respecting the engine has a lot to do with it. Slow in, fast out.

Mike
Author:  Monaco [ Thu Mar 17, 2011 8:44 pm ]
Post subject: 
So what do you have to change to run an A+ crank with the radius? :?
Author:  drmini in aust [ Thu Mar 17, 2011 8:51 pm ]
Post subject: 
owen1975richard wrote:
So what do you have to change to run an A+ crank with the radius? :?

My mate Dave Gray ran a new one of these cranks (from Minisport UK) in his Nb race car. I asked how long did it last, he said "not long at all"... :cry:
Author:  GT mowog [ Fri Mar 18, 2011 5:38 am ]
Post subject: 
drmini in aust wrote:
owen1975richard wrote:
So what do you have to change to run an A+ crank with the radius? :?

My mate Dave Gray ran a new one of these cranks (from Minisport UK) in his Nb race car. I asked how long did it last, he said "not long at all"... :cry:


Owen1975richard, what block & rods are you using?

drmini, I think your mate did something seriously wrong, or maybe it was where he sourced his crank from.
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