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Richie-Van-GTi

Another Machining Question

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Richie-Van-GTi

I need to take 1 mm of my pistons. Im just wondering, can this be taken from the conrod where the big end shell sits?

Im wanting to save a few quid as if I get the piston tops skimmed off then it also means re-pocketing the pistons. Please advise :)

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pip470

The pistons wont need re-pocketing, Your taking material off so the valve is going to be no closer to the piston than before. Not sure why you need to take 1mm off, maybe compression, if so take it off the piston. Taking some off the bottom of the conrod will achieve nothing. And i think your on about taking it from between where the conrod splits and you put the shells, If thats the case thats even worse, the big end will not be round anymore, more oval, and the bearings wouldnt fit.

 

Sorry if ive got the wrong end of the stick. Need some more info. Cheers phill

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ORB
The pistons wont need re-pocketing, Your taking material off so the valve is going to be no closer to the piston than before

 

This is true.

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Richie-Van-GTi

I should explain further, the pistons are being machined as the engine is restroked and is currently running 2 head gaskets so the pistons dont foul the head. If I take the material of the top then they will clear the head with one head gasket but I will also have to re pocket the pistons so the cam can still be run at its full advance. If I took the material from the bottom then the whole lot would drop the same amount and there would be no need for the extra work. Obviouly doing the bottom would mean not only cutting the curve face back but also the mating fances where the bolts sit so it would still be a semi circle.

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pip470

If it were to be a true circle and round then the radius will be the same all the way round, if you take the material from the mating faces where the bolts go the radius at 0 and180 dgrees will be less than the radius at 90 and 270 degrees,

 

im sure there is no way of getting the piston to drop down from the bottom end, its dictated by the crank throw, conrod length and piston pin hieght, I know im not being very helpfull here, Is here any chance of a drawing.

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Rippthrough

You'd have to get shorter rods as you'd just end up splitting the rods where the bearing lands merge to the beam. It'd be too weak.

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Richie-Van-GTi
You'd have to get shorter rods as you'd just end up splitting the rods where the bearing lands merge to the beam. It'd be too weak.

 

Thats the kind of answer I needed. I wasnt sure how much effect there would be on the overall strength and also if there was indeed enough material to go this route. Looks like I need to find some rods that are shorter by 0.5 to 1mm or pay the extra and have the piston tops machined. B)

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pip470

conrod.jpg

 

 

I just want to clear up where you were thinking of taking the material off. Cheers phill

Edited by pip470

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Mikey S
Thats the kind of answer I needed. I wasnt sure how much effect there would be on the overall strength and also if there was indeed enough material to go this route. Looks like I need to find some rods that are shorter by 0.5 to 1mm or pay the extra and have the piston tops machined. :lol:

 

 

that maybe detrimental to the strength of the piston crown itself if you remove that much. im not saying it cant be done, but you will have to be extremely careful with pre detonation control if you go that route.

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Richie-Van-GTi

pip yes that is where I was thinking, the red faces you have arrowed and also the semi circle where the shell sits so that it stays in a true shape.

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pugman211

What about the XU5 or maybe XU7 rods??? I've read a few post in other topics lately of them being useful. XU5 being 150.0mm i think, not too sure on the rod length of a XU9 thought sorry.

 

Just a thought.

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Beastie
pip yes that is where I was thinking, the red faces you have arrowed and also the semi circle where the shell sits so that it stays in a true shape.

 

 

Although that's possible, I agree with Rippthrough that there is a danger of weakening the rod. However - just consider what is involved if it were possible:

 

The rod needs setting up accurately so that the butt faces can be machined dead true in both planes. Once the sharp edges have been removed and a new lead cut in the entry of the threads then the cap is assembled to the rod. Next the whole rod is clocked accurately so that the new big end vertical center is produced in the correct place. The rod must be clocked in the lateral plane very exactly since there is no measureable quantity of material to remove adjacent to the butt faces. The rod is then machined to an undersize so that the big end parent bore doesn't quite "clean up" Finally the rod is honed to the correct parent bore size using a Delapina hone or similar.

 

Wouldn't it be cheaper just to machine the piston tops if that's possible?

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Crazy-legs!

Another option, if the correct length rods are not available, is to get the small-end of the rod, offset bored and re-bushed. This depends though, on how much material you have around your small end. BUT by shortening you rods, you may mess-up your bore to stroke ratio.

 

Hope this helps, Marc.

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Beastie
get the small-end of the rod, offset bored and re-bushed.

 

Aren't the pins shrunk into the small ends without any bushes? If you make them fully floating then you'll have to machine pistons to take circlips and then shorten the gudgeon pins as neccesary.

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James_R

use s16 rods and pistons? they sit 1mm lower down the bore than the XU10R(S) rod/pistons do and have a floating small end :) oh and bigger valve cut outs.

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mbayley77

There are two engines being broken in the for sale section i believe!

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Mattsav

As Beastie says, its far easier and cheaper to have the valve pockets machined than to machine the rod.

 

Machining rods is a pig of a job on a good day.

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