offset machining conrod big end... ??

[ed]

ok folks,

ive got a bit of a kooky idea to get some rods cross breeding into another series engine

its all really damn close in terms of the rod dimensions etc EXCEPT for the big end width which is about 0.070" bigger on the planned replacement rod. normally this wouldnt bother me much as surfacing the thrust faces of the big end 0.035" on each side would make 4/5's of no difference to the load bearing or integrity of the rod...

HOWEVER, things are complicated by the fact that in this particular instance, the 0.070" must be taken off one side only in order to create an offset of the overall rod centreline of 0.070"

looking at it, it COULD work ok in terms of the 0.070" depth of machining not really cutting into the guts of the rod too much, but its going to be real damn close. the new thrust face would sit right up against the machined recess for the head of the rod bolts. it also increases the side surface area of the thrust face, but only slightly, i doubt increased friction would be measurable by much,,,

the main thing im worried about is how im cutting into the 'fan' of the rod beam as it flares down and out into the big end. obviously this flaring will no longer be symetrical. will this assymetry introduce unwanted bending forces in the bigend? will the load still simply go straight through into the bearing as usual but simply have less area over which to work? is there some ungodly idea im missing here?

the machining will not be the end of the work the rods will get, so keep in mind that any harsh edges will be burred, the whole lot shotpeened etc...

this is going to be a fairly heavy duty cycling assembly so im not keen to do something totally whack and half cocked, but at the same time, im curious as to whether this might actually be just fine.

pics below demonstrate the rod big end that im talking about, with a rather fat red line painted over a scribed line i marked at the intended level of resurfacing. the red line overestimates the depth by a fair bit due to how fat it is. but it gives you some idea.

thoughts?





cheers
ed

[oldcorollas]

when rod is pushed down, i don't think it will have much of an issue... the force pushign bearing down will go thru original line? but the reaction force from the bearign will be offcentre (more on one side)... so will that cause the rod to push sideways a little?

when rod is being pulled down, the actual clamping axis is off-centre... will that induce a bending moment into the bolt?

then again.... assuming you are using the original crank, then the big end and little end centrelines will still be aligned no? if the big end little ends are alinged still, but centreline of rod is off.. there may be some bending induced... but if you shave the big end, the little end also needs shaving? or??

as for the friction surfaces, they can be relieved to reduce the actual contact area...

intuition says it is a little funny, but i spose it depends how over-engineered the rod is, in terms of being able to take non-symmetrical loading (but you could model the change fairly easily)

edit: i have no idea.. just guessing

[-GT-]

Hmmm, my initial reaction was "hell no!", mainly for concerns about balance and the sorts of loads going through it, worried you'd be weakening the rod. When you think about the sort of speeds/accel, and the fact that a lot of design has gone into the structure for the purpose of strengthening, i guess its also being wary that are you willing to risk it? How certain do you need to be before you decide your engine is worth more than the experiment?

Looking at the photos though, it almost seems do-able... Biggest concern is reduced surface area spreading the load increases stress in material? Or the imbalance caused? Are rods perfectly balanced along their centreline??? Are they symmetrical?

I think you've got a tough one here, how much safety factor would this eat into? How much can you eat into? I reckon the only way to get an acceptively conclusive answer would involve a lot of serious calcs and computer modelling... hmmm, who do we know could do that?

Otherwise, all you'll get is a lot of speculation. Which, looking back on all the questions i just typed, will probably be more than i've contributed.

Got me thinking though...

Is it only just that raised "lip"? Looking at the 2nd picture, it looks like that is raised to take the dimensions out, not necessarily for strength.

[-GT-]

Bah, beaten typing again!

So, are you willing to try and let us all know how it goes?

[ed]

the 'centreline' of the rod will remain the same. small end will be untouched. piston will centre be directly over the small end, and act along the middle of the long axis of the rod. the bolts (ie clamping) will also be dead centre. the whole issue is that is that one side of the big end needs to be machined down to keep all of the above in the centreline...

[ed]

Is it only just that raised "lip"? Looking at the 2nd picture, it looks like that is raised to take the dimensions out, not necessarily for strength.

basically yes.

[oldcorollas]

here's a question..... does it actually eat into the bearign area at all? or will the same bearings be able to be used....

if the original bearign area is maintained.... and the force from the piston is still transmitted along the rod centreline... (ie, the centre of big end bearing and little end bearing are atill aligned)... then the force on the bearing will still be centred also....

although the metal outside the bearing area does some of the work.... if you don't touch bearing, the actual loaded surfaces have not changed.....

when rod is pulled down, the force will still be centred from bearing, and be put into the bolts as usual... if the end cap was put in bending ... then maybe the change in length each side of bolt changes the leverage ont he bolt... but in reality, that doesn't happen..much...

if the bearign area is maintained, then at worst you are changing the mass/inertia of the rod... but maybe not enough to make a massive difference.... you could work out the amount of mass change by estimating the area (take picture from above, crop to appropriate sized square (based on the ruler in the photo), paint the metal area to be removed as black, then threshold the pic and see % of area that is black then multipl by thickness and 7.8 for iron)

[ed]

nope, doesnt touch the bearing *just*

[ed]

if the end cap was put in bending ... then maybe the change in length each side of bolt changes the leverage ont he bolt... but in reality, that doesn't happen..much...

all offset rods i have seen for V8s have the bolts in the centreline of load, which may not corrsepond to the shape of the bigend. so im guessing the bending moment is nil once assembled.

ed

[oldcorollas]

she'll be roight then

[30psi 4agte]

fill me in a bit more.

Why must the rod be offset ? From what i have read it sounds like the offset is also in the rod that you are replacing ??? ( hence the reason you need to make the replacement rod the same ??

[kingmick]

all offset rods i have seen for V8s have the bolts in the centreline of load, which may not corrsepond to the shape of the bigend. so im guessing the bending moment is nil once assembled.

ed

correct! you wont have a problem as long as the rod has the meat to do it, which it should. ive used offset roads in V6 turbo running big boost and plenty of atmo V8's.

[ed]

it sounds like the offset is also in the rod that you are replacing ??? ( hence the reason you need to make the replacement rod the same ??

correct interpretation

[JustenGT8]

Mate had a similar issue with the 22R crank into the 18R block. Had to side grind the rods just over a 1mm so not too different to what is required of the 7M rods. The 7M rods are probably a big stronger too so with no issues thus far from the 18R rods i wouldn't expect any from the 7M ones?

Apart from the extra stroke ratio are there any other benefits to the 7M rod? Does it look stronger than the UZ rod (the good rods not the wussie VVT)?

[kingmick]

hahahahaha wussie! havnt heard that for awhile!