I bent a standard rod due to running too much torque - they were balanced, but not shot-peened

Had no problems with thew forged rods (so far)

OK I have just spoke with Farndon who are making me up 35 rods for the group buy. He doesn’t know this engine but is working from the data I have supplied. He said that the Piston end clearance doesn’t matter as much its crank lead. However the crank end clearance is very concerning. To have a 22mm rod on a 26.5mm journal is a worry. His clearance is to usually run 3thou either side, he has not seen a clearance this wide from factory before. The mystery is that this clearance is OE spec???? Since the rods are at drawing stage I have told him not to go ahead with production just yet. His suggestion would be to widen the rod however he doesn’t want to take responsibility in case the engine was designed this way for a specific reason. I was totally unaware that a total gap of 4.5mm+ from factory would be a concern so now I’m concerned. It’s easy to say widen the rods but the real question is why have Fiat done this is there a genuine reason? Would a wider rod cause a problem? Let’s get hold of the boys from Brazil and see what they have done?

For the rod to be walking on and off the oil wedge as described would suggest that there was insufficient oil pressure in the first place. Given the pistons have forced gudgeon pin oiling and we have the benefit of oil jets I cannot see how you should ever not be on oil film, and in any case if there is that scenario regardless of design the little end will last about 5 minutes. I actually have a piston which was swapped after 20k miles on mine, there is no wear on it there whatsoever. There is a really minor scuff on the skirt from a foreign object but thats it.

J i've not seen an engine without thrust washers somewhere on the crank - I don't think its for the conclusion you made, as the washers have to take up the pressure of the clutch release bearing pusing or pulling against the crank.

I checked 16v OE - BE bearing width 19 ish mm, rod big end width circa 26mm, piston pin to boss clearance 1mm+ each side.

Here is a pic of that used piston I mentioned 2.4 piston after 20k miles You cannot see any wear on the piston to rod interface because there is not any...

Yes my mistake Rich, I meant floating pin not thrust washers (what was I thinking ).

Due to further thinking it has to be lead by something. With the OE piston small end being 0.3mm clearance (going by Stichl measurements) and the big end being 4.6mm clearance it would seem this engine is piston lead as standard, I can’t see why as it has a floating pin, not pressed, then piston will become damaged as standard??? Its gotta be crank lead. I still think this engine is crank lead despite the 26.6mm big end width. I’m gonna go a measure a standard piston 2morrow as I think the 0.3mm (measured by Stichl) sounds too tight. I may be inclined to increased the rods width at the big end only as 4.6mm is sloppy however I still think there must be a reason for Fiat doing this.

Whether the pin is fixed/pressed or floating doesn't have a lot of bearing on the lateral clearance between the rod and the piston to be honest. In the CP Piston design usually there is forced gudgeon pin oiling through oil relief holes unless this option wasn't chosen in addition to the oiling from the jet. The rod should centre anyway which is why even on OE setups that are rod-guided there is a reasonable gap, much more than would be designed in if there were any sort of substantial lateral forces operating on the rod. Otherwise there would be a full bearing face and a suitable bearing material would be utilised.

anyone with some influence in motorsport world who can send email directly to Torino to ask this?

just a thought..

I am still on the side that there is a reason Fiat have done this. It hasn’t caused a problem in the past and looking at JohnS pistons after 20K I don't think it will be a problem for the future either otherwise we would have blown engines all over the place, so I think I’m gonna stick with the OE design at 22mm all the way.

Some of us have - five! Three of those include 2 snapped rods and one big end failure. Keith's machine shop identified an issue relating to the way the rods sit on the crank (remember, I don't do technical) a while back. Whether my failures are related in any way to this, who knows....

But my current preparer has a professional race engine builder looking over the 2.4 and he's not happy to have his name associated with it. So I await his proposal for a solution

And Fiat Italy will care because?????

What about the 16VT engine does that run a similar setup?

16vt bears no relation to the 20vt and many more people with race experience have been working on them. I know someone currently making some really trick bits for 16vt engines but this problem has not been mentioned

Just commissioned a set of Arrow rods to my engine builder's dimensions to fit my 2.4 without this issue. A long way from cheap but they reckon they can turn them round in 10 days

Funnily enough I'll have a set of 2.4 rods for sale in a couple of weeks, assembled, then taken straight back out again ;-)

So what big end width have you gone with then?

Why ask FIAT?
every engine manufacturer all over the world has to decide either to lead the rod by piston or lead it by crank.

FIAT decided to lead it by piston. That's all.

If someone has forgotten to tell CP or other manufacturers about the fact, FIAT is using piston lead rods, the above mentioned st happens.
C.P.S. is a piston manufacturer from Italy and that's the reason why my pistons got the right design, they know the rods are piston lead!

For my CP-piston/Cunningham-rod combination I will use aluminum spacer washers as Stichl said already.

But, and this is very important for everybody who is concerned, there will be no problem at all as JohnS told you. Nobody needs to dismantle his engine to insert those washers. But building up a new engine, it would'n be a good idea to forget this discussion. I will sleep better with the washers inserted.

Hartmut Lohmann

I have no idea! On the basis that I don't get a dog and then bark myself, I have engaged a race engine builder to resolve this and in the first instance getting his resolution organised was more important than worrying about what it is!!

But sometime over the next couple of weeks I will ask the question. Obviously I will be able to supply similar rods to anyone who wants them but it seems the existing ones are working OK for road engines (that seems to be the consensus amongst those already running 2.4) but the Arrows rods are REALLY dear. Farndon would have been able to do some cheaper, but not if I want to race this season - slight exaggeration - as with the existing group buy, would probably be looking at 10-12 weeks lead time. I don't have that

Forgive me if this sounds uber stupid.....

But would large side to side movement in the crank (i.e clutch pushing it) not create the exact same problem that is occuring - regardless of whether there is a large gap at either little or big ends (ofcourse the crank would have to move a certain amount in order to move the rod, which then grazes inside of the piston.

Chris

Your correct but its only minor amounts something like 0.3mm

There are a few things that need to be clarified here

For all the engines that i have worked on and examined in depth, all the Piston led engines have always had pressed pins, with this i mean the piston pin is pushed through the piston then through the rod then back in to the piston, the pin will be held in place by a friction fit in the small end of the rod and the bush part is then in the piston rather than the small end

This is a non floating small end

The reason for this is no doubt many fold but one i know for sure is to locate the rod and stop it from walking up and down the journal, and to stop the small end from wearing on the inside of the piston boss, hence being called piston led

Now we know for a fact that the standard setup is a fully floating setup and that would suggest that it is in fact crank led

But in the same breath, the clearence either side of the big end is massive and that would suggest a piston led setup, especially as the small end clearence seems to be smaller than that of the big end

Now obviously this is only my view and understanding of the Fiat 5 pot setup and is by no means gospal


So i put forward that this engine is somewhat of an odd-ball and seems not to follow convention, there a couple of things i do know for sure though

1, many people are running with forged pistons/rods with large equal clearence at the small and the big end for many many miles and no problems at all

2, and this is just my opinion but im sure many people from a engineering back ground would agree, and that is that its seems bad practice to use the inside of the piston as a bush for the rod to run against no matter how breif it may be

Now i have not worked on every kind of engine out there so if someone knows of another engine that is piston led and still fully floating at the small end i would be interested to know and it may be able to shed some light on the Fiat setup here and why they have done it like this

I just found an offer of the 20 years old titanium rods used by Mercedes in the German DTM

They are piston led

ebay no. 260362270042

Yes these are exactly as you discribe, and they are fully floating

here is a link of the above to save other people putting it into ebay

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?V...ries%26_fvi%3D1

But one point i must raise is that these rods have bushes either side of the small end, these are steps taken to stop the rod wearing on the inside of the piston boss.

Fiat have not done this, why? well it would normally suggest its a crank led engine but that seems odd especially with the large clearences at the big end


I take it these are from the old Cosworth 190E DTM cars, do you know if the standard engine run the same setup or was that a pressed fit pin?

the only thing i can confirm is that the 190e cosworth engine as standard had oil feed up the rod to the small end and these titanium ones do not, so im not sure what they have done about lubricating the small end

I cant find any other details on it myself

ok i have found a workshop manual for the 190e engine, its quite clear its piston led but this manual states that the pistons have thrust surfaces cast into the piston boss for the rod to run up against, I cant remember seeing a 20vt piston haveing anything like this but we need a picture of the inside of the piston to be sure, can anyone help with that?

There are partial surfaces but not a complete thrust surface because it isn't designed to be a load point

GC sent me this.

“I've read that whole thread. Generally very constructive and interesting insofar a thing called a piston-lead rod is interesting to me at all, because whilst it appears to exist (and I freely admit I had not heard of it before esp on a full-floating setup) it is utterly loony, risky, expecting an aluminium piston to restrain a steel rod. I cannot think of a single good engineering reason to do that. That said I don't get much time to think these days.

The fairest thing to say is this:
If you bring me a bare clean block with caps and bolts, crank, main brgs set, one big end brg pair (good used will do) and one complete conrod (std) and a sample new piston of the type your rods will be for (forged I assume) I will evaluate this and tell you what I would do.

I have to look at the grind width and radii on the crank, bore to crank relative positions, offsets blah blah. This is commonplace for me and it involves dummy building the crank into the block and setting up a rod piston set to see what lies where.

When I see the layout I will know but I cannot second guess this because, as is so often here, I have no prior exp of that bottom end albeit I have plenty with engines generally.”

GC

I guess the only way to get to the bottom of this is going to get a professionl engine builder design the rods/pistons.

My thoughts entirely. I've gone halfway house on this due to issues with both time and money. I'm having a set of rods made with appropriately wide big end, but even then it was suggested to me that a 19mm gudgeon pin is rather small for a turbo engine - at least in a race application. If I was to build another (please, NO!) it might be better to have the pistons done at least with larger gudgeon pin and rods made to suit

if you were going to through so much effort as to practically redesign the piston and rod you may as well get a custom crank done at the same time, especially in the case of the 2.4 setup as the rod to stroke ratio is far from ideal, but that's a whole new set of discussions and problems

Today i had a discussion with an expert of rally's engine at my job. We talk about rods and big-end bearings...

J you told me yesterday that fitting 26mm rods (not original width) with 22mm big-end bearings (original width) won't be a problem... but the "expert" at my job said it's not possible

He said if we ask Farndon to make 26mm rods we will have to find big-end bearings that are specially made for the new sized rods. (larger bearings)

Personally, i think it's a coherent explication.

So where is the truth guys?

Can someone which have widened rods told us if widened big-end bearings are needed?

The rod can be made to wider but still use O/E bearings but there are other problems that guy croft has raised, the main one is wether or not the crank journal for the rod is in the centre of the bore or of set, TBH I think they should be left alone at 22mm, there are many coupes running with this setup and no problems

hmm, I've been following this thread with some real interest.

I am also baffled at the idea that there is that much clearance on the big ends & that the rod is being centralised on what looks like both ends ( crank & gudgeon, are you not sure that the gudgeon pin holds the small end in position with some internal circlips ? normally the piston is a loose fit on the gudgeon & the pin / rod is the interference fit )

unless the crank endfloat was to suddenly change ( eg main shells failing etc ), it does seem daft.

@ John / Tipo, did you use a new oil pump when the 2.4 was built ? ( silly question I know ).

My coop when it get VERY VERY hot, drops to about 1.75bar at idle, but it's now set to about 700 rpm, so it will be quite low, however as you rev it, it's spot on.

I can only assume that there must be some mechanism that prevents the rods from walking across the crank journal ?

usually, the rods are just smaller in width than the crank & the big ends are usually shaped to use the radii of the crank as its centraliser when oil pressure builds up.

Must admit, I've had no probs ( yet ). I know Trev has some interesting views, but alas, a my coop won't be tracked, it'll prob be ok ( fingers crossed ).

Ha! I have no views, I don't understand! Its all gone over my head - had to get Rich to translate for me ;-) But having entrusted the kappa to a motorsport outfit, I have to go on what they recommend or there is no point using them

Thing is, until someone breaks a 2.4 and can put the issue down to this, then I don't see any reason why anyone already with one would strip down their engine

that is true pal!, just hope I'm not the one to encounter this. I'd either scrap my coop, or just go back to a boggo 2.0 with all the forged bits.

As you said Trev, your engine builder has more experience that most of us on here, so his advice & guidance should be noted