25 replies to this topic

[tom1]

If I remove the connecting rod pin by pressing it out , can I reuse the piston and pin. I know I will have to heat it up to refit it ?

 I need to remove it so I can take about 1mm 0.040" of the top of the piston to reduce the compression ratio . It currently stands at 11.72:1 (a bit high I think on a 74mm piston) . 1mm would take it down to about 10.48:1.

[ACDodd]

Perfectly fine to re-use as long as there is no damage when you check before refitting. You need a proper jig to hold the piston or it will be likely break.

Ac

[Cooperman]

By the way, when re-fitting you don't heat the pin up, you deep freeze it and heat the end of the rod.

[sledgehammer]

When you think about it - that pin gets a lot of force on it

 

& so does the small end of the con rod

[KernowCooper]

+1 on watching the piston during removal/refitting, I served my apprenticeship in a Vauxhall dealer and the 1256 engine were all press fit we had a special tool to support the piston

[Cooperman]

It might be possible to skim the amount you require off of the piston top by setting the rod & piston in a Bridgeport and milling the piston top. It would be safer than trying to get the pin out without damaging anything. Could you do this Adrian?

[tom1]

It might be possible to skim the amount you require off of the piston top by setting the rod & piston in a Bridgeport and milling the piston top. It would be safer than trying to get the pin out without damaging anything. Could you do this Adrian?

I like this idea.   I know a man with a Brideport

[Cooperman]

Thinking about this some more, you could make up a hardwood cradle to hold the piston(s) by turning a hole in a piece of hardwood, cutting the wood in half and cutting back the faces of the wood so that it clamped the piston almost right around its circumference, then mounting the cradle so that the piston is horizontal with the face to be milled accurately vertical (this is critical) and end milling the piston face with a fly cutter . A really good Bridgeport guy should find this no problem and it would take away the risk of damaging the piston by trying to remove it from the rod.

A tolerance on face to gudgeon pin centre of maybe +/- 0.002" would be ok.

 

A cheaper alternative might be to open up the chambers in the head by a couple of cc, although taking 0.040" from the piston top is like increasing the combustion volume by just over 4 cc.

 

11.7:1 is a bit high, but anything up to around 10.7-ish should be acceptable so long as you use premium petrol with octane booster and have the correct advance curve on the ignition.

Edited by Cooperman, 24 June 2014 - 11:04 AM.

[ACDodd]

I remove the rods from the pistons to machine. I also have a fixture to remachine the hemispheres too.

AC

[tom1]

Anyone want to check my math, before I find  a wood turner,or contact AC Dodd

 

ring to land volume          0.21

volume above piston       4.38  (1.02mm)

Head gasket                    3

head volume                   24.8 

stroke                             81.3

boar                                73.9

 

I make it   11.76:1 and I would like to run about 10.5:1. so would a reduction of 1mm (0.040"approx) do the job?

[ACDodd]

Hmmm, what deck height are your pistons at TDC? What Pistons are fitted? What dish? What head gasket are you using?

[tom1]

Deck height  at TDC is 1.02 mm(below). I think that the pistons are Ford ,back from the early 90's, it is a flat top( like my hair in the early 90's). I was going to use a shim type from MED (just down the road) unless you think there is a better way?

[ACDodd]

The fiesta pistons have a pin diameter which is a few tents smaller than stock. Therefore careful measurement will be needed before refitting to ensure they will not fall out in service. The other point here is the crown thickness. These pistons are well known for there fragility after these mods. I would suggest looking for a suitable 74mm piston replacement with a dish. More reliable and sensible way forward. You will also likely need the block honed to suit the new piston.

Ac

[ACDodd]

Look at minispares c-str314. A dry build will be needed and these May need a few thou taken off the top before use.

Ac

[tom1]

Out of interest, would you know which sort of Fiester piston  was used back in the day ?