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RB26 crank modification

ylwgtr2

By ylwgtr2,
in Victoria

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ylwgtr2 Grand Master

ylwgtr2

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well i have pulled down an RB26 for rebuild and found a knackered crankshaft........new cranks are priced quite resonable so i bought 2 new cranks one for the engine and one for myself....heres the mods that i did on one of them today.......first we set up one end in a 4 jaw chuck,and we put a bearing on the other end and clamp that in a steady rest .....we check its true by placing a dial indicator on the main journal surface and spin the crank in the late....we then adjust it until its true..... then we start machining our first crankweb counterweight,we take it from as close to the rod end journal as we can get it right to the main journal at around a 50 degree angle(i cant remember exactly what the angle was)the working back toward the front end of the crank(you will notice only every second web is getting done,this is because every first web needs to be cut back to the opposite end of the crank,we do this by reversing the crank in the lathe)i managed to get all of the

second webs done today....i will have the first webs done tomorrow then start balancing it...heres the pics

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https://www.sau.com.au/forums/topic/51675-rb26-crank-modification/
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inark Grand Master

inark

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... i have no idea what ur doing... or what it does... but it looks damn impressive :P any pages u can refer me too so i know exactly what this does? or wanna give dummies like me a run down on what this does overall in the way of performance???

Laurence Mentor

Laurence

Posted
Posted

If you make things that spin around lighter, they can spin faster, or more easily, unless you made them too light because then they break. Also if the spinny thing has to sploosh through oil it is easier if there is a nice edge on it.

I think ylwgtr2 should probably post a pic of the bucket of metal filings that must have come off those cranks to give people a better idea.

Snowman Grand Master

Snowman

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... i have no idea what ur doing... or what it does... but it looks damn impressive :P any pages u can refer me too so i know exactly what this does? or wanna give dummies like me a run down on what this does overall in the way of performance???

I have this problem just about every time I speak to troy! LOL

He's always doing something that is way beyond my comprehension - I just have to nod and presume what he's doing is a good thing!

inark Grand Master

inark

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I have this problem just about every time I speak to troy! LOL

He's always doing something that is way beyond my comprehension - I just have to nod and presume what he's doing is a good thing!

its amazing how a lot of people seem to get this issue when troy speaks :P

troy... from now on i reckon u need to dumb it down a little for us.

ylwgtr2 Grand Master

ylwgtr2

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... i have no idea what ur doing... or what it does... but it looks damn impressive :P any pages u can refer me too so i know exactly what this does? or wanna give dummies like me a run down on what this does overall in the way of performance???

ok......gojira is pretty much on the money....."rotating mass" is the word for today.....Ever fitted a lightned flywheel?(andrew? ;))Its the same deal with cranks,pistons,rods and any thing thats rotating or reciprocating...the lighter it is the easier it will move and the less force it will take....therefore more response however there are trade offs.....example you will find that an engine that is made for smooth power will have heavier crank assembley(balancer and flywheel included)For take off heavy crank assemblys are good eg an idle no accelerator take off is easy......but try doing is with a light set up and you will most likely stall....This is because of the "stored energy" in the heavy setup is greater for obvious reasons....ITS HEAVIER!So in laymans terms it takes more grunt to drive a heavier flywheel/crank assembly but our aim is not to sit at constant speed...its to get it to the point of where everything happens in a hurry!Hope that makes some sense.....Oh and messing with the crank doesnt really cause them to break......its poor balance that usually does that...if you have a look at my doctored pic you will see the red line....thats the part that takes the stress i have only just taken the cast marks out of that.....the real weight has been taken off the crank webs on the opposite side.....just spinning the crank up it seems so much more "right"......you can see the crank doing "its thing" in the last pic

ylwgtr2 Grand Master

ylwgtr2

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oh and in the 3rd pic the left hand side of the conrod journal has been "done" and the right hand hasnt been done yet so you can see there is no material taken off the the "Power transfer section"of the crank

ylwgtr2 Grand Master

ylwgtr2

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This is a great thread! keep up the good work :P

Im glad your enjoying it!.......In regards to gojira's "oil splooshing"comment this engine will also wear a "crank scraper plate"Which runs a tight clearance between it and the crank edge....this removes any oil from the crank to stop it flinging all around the show inside the engine block....more on this later

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