Race Engine Forum UK

Hi Guy!


Just recieved a set of custom made forged pistons from a reputable source, and have a general question regarding forged pistons:

The piston wrist pin fit in the forged piston it is "loose", when comparing to the pin fit in std Fiat pistons.
Actually it just slides freely in the pin bore while the Fiat needs a bit of fingerpressure to move.

The piston pin from the forged set has been "trial fitted" fitted in a std Fiat piston and it needed a bit of squeeze....
...so dimensionally the pin is identical in diameter - as it should be.


Is this loose fit OK?
If so is it related to piston thermal expansion rate?


Best regards

Remi

This is very hard to quantify long-distance but I'd forget comparisons with the OE piston.

A race pin & piston combo should have no more than a 'sliding fit' betw pin and piston bore. So (depending on the boring accuracy attainable at the factory) there should be no more than a fraction of a thousandth of an inch clearance on diameter there. The pin should just fall thru the bores under its own weight and there should be no evidence of vertical movement at all when you get hold of it in position and give it a good shake.

If the pins are loose they will cause a lot of damage. 0.866" is typically the pin diameter needed but if they are at that size and too loose by my standard above there is certainly a problem with the pistons.

If in real doubt send one to me.

G

I assume you have tried all pins in all pistons? i.e they are all the same.

Hi!


I am not overly worried about the pistons, as its origin has a very good reputation and the pistons look/feel very nice by inspection - I ask more of curiosity as the piston pins move in a way I am not used to.
There is no "slack" or clearance issues at all on any of the pistons. The reason for asking is that the fit is very different to the cast pistons I have been working on so far (Fiat/Mahle/Mondial) I am a novice to race engine building to be honest..... road engines I have done a few.

Nobby wrote:I assume you have tried all pins in all pistons? i.e they are all the same.

Hi Nobby!

The pins/piston fit are all equal, and the pins has the same feel to it as the oe items when fitted to a Fiat piston - that means the dimensions are correct and consistent.

Thank You for Your interest!!

My grandma always told me that the easiest way is to do it right the first time - I am still trying!!


Remi Lovhoiden

Could this have to do with the expansion rate of the metals used? Cast piston alloy tends to expand at a different rate than the alloys most often used for forged pistons. I'm not certain if there should be a more loose fit, or more tight. It might actually be that it has to be even tighter, since the forged pistons tend to expand more, making the hole bigger when on temperature? Maybe the pin material is the key here? Or is this line of thinking totally wrong?

hi all,

Yes there are differences in expansion coefficient between different alloys, but those differences are very small compared to the difference between aluminium(s) and steel. I don't see a reason there.
(BTW: holes grow as objects expand, they do not close up as is sometimes reasoned.)

What does matter is the quality (roughness) of the bore surface. Given that the tolerances for a 22mm sliding fit will be somewhere around +0,005 to +0,01 mm for the bore and -0,005 to -0,01 mm for the pin (giving 0,01 to 0,02 mm play) or even less, we are in a tolerance field that borders on values for surface roughness which is measured in micrometers.
Here a smoother finished bore will appear wider, but in reality when judging the rougher bore you will only touch the tops of the 'mountains' that make up the material's surface. Flattening those mountain tops is part of running in. A mass produced item will have its production tolerances adjusted for this.

I can very well imagine custom made pistons having a better (more expensive) bore finish that is already near nominal running clearance. It will need less time for breaking in and last longer (more material bearing the load).

regards
Tom

Some very good points made there Tom, stuff most of us would not think of..

Well done!

G

Hi Guy!


When You offer to inspect my pins/pistons - I just really cannot resist....

Expect to recieve a parcel soon.

Thanks for teh response everyone!!


Best regards

Remi Lovhoiden