Lightspeed who are rebuilding my engine are concerned about the amount of sideways movement on the Cunningham rods I have on the 2.4 engine.

The width of the big end of the rod is 22mm. Ideally they would like to fit a 26mm wide rod. However Cunningham have a 12 week backlog of work. The alternative, which they they say is a valid solution, is to put high tech washers on the piston pin either side of the small end to centre the conrod.


The previous engine builders also commented on the amount of slop the rods had. I spoke to Barbz about this issue and he said that the rod design was the same as the original Fiat ones and the amount of movement was expected and was part of the design of the engine.

Any thoughts.

John

22mm is OE width at big end and small end of the rod. Crank is just over 26.5mm. The piston boss is 26mm. I can't see why this would be a problem? This clearance has been designed in by Fiat for a reason and it hasn’t caused a problem in the past I would leave it well alone.

John if it bothers you please send an email to Guy Croft and he'll explain what you need to know, its a floating pin and I'm darn sure my set up is the same.

it's nothing to worry about, tbh I can't think of any engine I have worked on that runs much less of a clearence gap either side if the small end, the big end is much more important and will stop the rod from sliding up and down the journal, they are worrying about nothing

The 20vt engine is "piston guided" (in German language)- means the rod is guided / centered by the piston. Therefore there is almost no gap between an original piston boss and the end of the rod.
There are other engines which are "crank guided" (not the 20VT) - these engines then will need more gap for the piston boss. The 20VT gap between piston/rod should not be more than 1mm. The original piston can be measured with about 23mm in the area of the rod. Therefore I assume that your piston is not correctly designed?!
Juergen

Lightspeed have never seen an engine designed like this before!

What they are seeing is roughly a 3mm valley either side of a 19mm bearing surface. The shell for the big end is 19mm wide. However the rod can float either side thus loosing roughly 20% of it's contact surface. There is evidence on the inside of the old pistons that the small end has worn grooves into the piston at either end of it's lateral travel.

what pistons do you use? the original ones have a gap of max 1mm to "hold" the rod in the middle...

What information is needed is, the width of the rod at the small end and big end, plus the width of the big end journal

Then the measurement of the inside of the piston between the piston pin boss's if you know what i mean

Stichl, There are examples of piston led engines that run much larger gaps either side of the small end for example the RB26 or even BDA's and BDG's that in some cases run large clearence gaps at the small end and the big end of the rod

edited to say the RB26 is in fact crank led, my mistake

Yesterday I checked the piston boss of my "old" 2,4l conversion pistons from CP.
It was not nice, what I had to realize...
The piston boss has a dimension of about 26mm as well...
The original one has about 22,3mm (measured)!!!
Means that the CP pistons allows the rod to move about 4mm (!!!)towards the original one (0,3mm!!!)
Maybe that this tolerance killed your shells?!
Sh... I installed the same pistons in my 2,4l conversion.
I hope that I won't get similar problems because of this "design problem".
But I think that Johns has the same pistons and rods in his car - and this already for a significant sum of miles.
I think, we would have realized already, if there would be a "2,4l conv. series problem"?!
Sh...
Juergen

The engine has thrust washers so there is no worry here as that is the most important factor. Engines with thrush washers are usually crank lead. I can't see any serious movement despite the clearance as the rotational force in an engine will keep the rod central anyway. Seriously John you have nothing to worry about.

Ah John I just re-read your message, I'll ask Guy for you.

rich

trevor are you reading this?

Yes! Give me a call some time

I've sent Guy and Barbz an email to try and clear this up as there seems to be conflicting comments regarding whether this engine is piston or crank lead.

Given the width of the big end is only 22mm compared to the gap of 26mm it sits in suggests it is piston lead. However the width of the boss on these pistons compared to the width of the small end would allow too much movement. It also appears that the width of the boss on the aftermarket pistons is wider than the OE pistons.

I had a look through the workshop manual and the only specification that looked relevant was "Gudgeon pin Small end bush" which is a width between 0.006 - 0.016. This is on page 87 (Page 10 of the technical data fot the 20V and 20V Turbo engines).

John

Guy says

"no way can you pack up the rods to reduce the lateral movement, any loose object there will knock the crankpin to bits. You need new rods that fit"

John

That's not nice...
I think even a shrank special washer becomes loose after some 1000 cycles and then will saw through the piston pin :-(
Sh... there are only 2 solutions
- dismantle the engine and change to another piston / rod - this is very expensive and needs a lot of time / work
- or to drive with this faulty design...

It seems to work well, when looking to Johns etc...
I haven't heard of any problems so far - and there should be more 2,4 Barbz engines on this planet...

What could happen when driving with such a gap?! Does somebody know possible consequences?

Juergen

Umm, so it looks like the rod is the same width as the OE item and it's the piston design that has changed. The boss on the new pistons (2.4 design) allows too much movement in the rod. The boss is 26mm wide compared to about 22mm in the OE pistons.

I'll get Lightspeed to comment on the wear on the piston boss and big end bearings but they've never seen it so bad. No wonder I had terminal oil pressure failure after only 1000 miles.

Lightspeed are recommending fitting some 2mm thick teflon washers to reduce the boss size back down to 22mm and have it piston lead. Either that or order some new rods with a 26mm big end and have it crank lead. Left as it is I'll be looking at another rebuild.

My problem is if I wait 3 months for the new rods the car won't be ready for the summer :-(

John

This is what Guy said in a further email:

My view - which is law at GCRE - if nowhere else, is that the rod must not be constrained by the piston, because to be so, the face of the small end would have to bear against the piston almost continually and it would rub it to bits. It would also generate a side load in the pin that would probably knock out the wrist pin clips too. I have never encountered such a setup. Every racing piston I have ever seen has a generous clearance in the span, albeit that it varies engine to engine according to speed and allowable bending load.

My design criterion with rods is that the clearance of small end in the piston span must always be greater than the clearance of the big end in the crank. What others do is often a mystery to me.

we need to know the side clearence on the big end as i'm sure it's lot less than what is at the small end and as such will not let the rod move to the extremes of the gap at the small end, does that make sense?

The crankpin gap is abut 26mm (roughly 19mm bearing surface with roughly 3mm valley at either end) which is the same as the boss gap on the 2.4 pistons. The rod is 22mm thick at both ends. Therefore it's possible for the small end to rub against the inside surface of the boss.

If this happens then the big end bearing is only sitting on about 80% of it's contact area.

As said earlier the engine is crank lead due to the thrust washers. All forged pistons fitted run a bigger clearance, the Accralites I have have a 26mm pin boss, they use a very similar forging in the Honda race engines. I have excellent oil pressure all the time. Even done a good 10,000 miles since the rebuild. CP pistons also have the 26mm boss. I don't see Flea, Nigel plus all the other users on here with forged internals having a problem. The piston end is not a problem and the crank end is still OE spec there is no concern here. I even spoke to Accralite this morning regarding this matter and he is in agreement.

Where do the thrust washers fit? Are there two pairs for each crankpin (one pair either side of the rod)?

Doh - found this out after posting above:

The engine had only one pair of thrust washers. These fit on the crankshaft to prevent the crankshaft moving too much.

No, no there are no thrust washers on crank pins, the 26.5mm gap is correct. There are two half cut washers in total that reduce the end float, they sit either side of the middle (ish 5 pot ) main journal. This is what I mean by being crank lead. John reread what GC said in your further e-mail, it’s correct. I think lightspeed are used to V8’s.

Have a look here:
http://img21.imageshack.us/img21/8023/dsc00532fg3.jpg
http://img21.imageshack.us/img21/5968/dsc00533iy8.jpg

Lets just read this and take it as gospel

"My design criterion with rods is that the clearance of small end in the piston span must always be greater than the clearance of the big end in the crank. What others do is often a mystery to me."

GC knows what he is talking about and what he is saying is the same as what i have seen in practice

To put your mind at rest there are a lot of cosworth BDA's and BDG's running a narrow journel setup that is very similer to yours without any problems, these are 15-25K race engines and so if they are good enough for cosworth/ford plus Acralite and GC then i think that pretty much covers it

Not to mention that loads of people on here that are running the same or similer setup are having no problems at all

The rod has galled the side of the pin boss on both sides and has worn the bearing in a very unique way. Usually a bearing is worn in a parralel manner but these bearings have wear grooves at a 30 degree angle. Which means the rod is walking on and off the oil wedge. It is my op that one end or the other needs to have minimal movement. I have no problem with a narrow big end or a floating top end just pick one or the other. I am glad GC knows what he is doing but "it is a mystery to Me- cunningham-JE and Wisco" why this combination has been designed this way. I understand now it is not GC who engineered this combination so I retract my previous statements. The wear patterns indicate that this is not a proper solution though. Lightspeed builds 20 to 1 imports to domestic V-8's. The actual engine builder has years of Superbike motor assembly and a solid reputation. This is a poor design and needs to be rethought the small end of the rod should not rub on the pin boss end of story. It is a new solution is needed. I think a thicker rod end is ultimately the best design however that has major time constraints on it. Teflon pin buttons have been used in HP engines for years the idea is to make a teflon washer for the small end. So if anyone has data that could explain why this is a bad idea I am all ears. Either way I need a solution because this motor is not going to be assembled with the previously engineered combination.

I have talked with my former F1-engine builder Hartmut Lohman...
He confirmed what Lightspd said:
This is no correct design...
The Italian forged CPP- pistons have a correct gap between piston and rod - they are correctly designed. This problem seem to have not all aftermarket pistons - so far we know this problem from CP and Accralite.
Hartmut has a similar problem with his rods / piston combination from a 2,4l Fiat Coupe engine and he will install High End washers made from a special Aluminium to center the rod.
It is not acceptable to have any gap between this washer and the piston pin - therefore he will shrink these washers.
He has talked to various other race engine buliders - they confirmed that often test engines (also in F1) use such washers to center the rod - therefore this is practicable solution...
But not for me - I won't dismantle my engine any more...
My 2,4l has seen about 10000km - without any problems. A check of the shells 3 months before showed that the crank and the shells theirselves are in great condition...
I hope that this will remain for the future, too...
Juergen

Didn't Nigel have problems with his rods not so long ago? I can't remember if they were standard (lightened and shot-penned) or forged ones though. Plus I can't remember what the problem was, bearings or a bent rod etc..

Anyone else remember? Nigel!

I think Hartmut has machined a flat face on the piston for those washers.

But they are generally a floating design not a guided design.

If you want to change something I suppose it is the piston not the rod that would change. The Mariani rods and pistons were the same btw.
For me I am not bothered because when my engine was apart for inspection during the 2.4 tests there was no wear there

Can we have some pics of this specific engine and its problems

Also why would you be suffering from this problem but there seems to be no other documented cases of this problems

Might this be the first time a professional ENGINE builder has worked on one of these engines rather than (with the greatest of respect) a more general mechanic or even home builder?

Delivered my 2.4 bottom end to a professional engine shop on Monday along with the head and cams so they could measure exactly how much freedom we have to adjust the cam timing once on the rolling road. I alerted them to this thread last night (as it goes over my head technically). They have come back today telling me they cannot condone this engine for race use. Maybe they will be OK in road use, occasional track day, but I need a race engine that is ragged 100% ALL of the time. I have asked the engine shop to suggest an appropriate solution, at the moment it looks like I'll be having a new set of rods made up that don't have this free movement