benlilly 0 Posted October 30, 2006 Hi, I weighed my pistons last week and found there to be between 4 and 9 grams difference between them. After weighing a slice of ali rod, it was pretty clear that 9 grams of metal is alot to remove from the inside rear of the piston (where I originally though to remove material) and that perhaps it was better to take the material off the flat where the gudgeon pin is located. This seems like a safe area to machine. The Gudgeon pin is 5mm back from the existing flat face and I think I should only need to take off 1-1.5mm from each side. Anyone done this and know that it is ok. Thanks, Ben Share this post Link to post Share on other sites
Guest ashnicholls Posted October 30, 2006 Dont know how good it is, but I balanced mine with pistons and conrods together and then took as much was needed off the rods. On the bearing cap as that had a lip, and on the rods. Because as far as I see it all you need is the downward wieght needing to be balanced with the upward weight. I am no engine tuner, but I can't see what I did doing it any harm anyway. Cheers Share this post Link to post Share on other sites
Banjo 1 Posted October 30, 2006 (edited) Hey, I don't know about the pistons sorry but i'm from Stroud too!! and my names Ben! Edited October 30, 2006 by Banjo Share this post Link to post Share on other sites
Anthony 1,002 Posted October 30, 2006 9 grams variation sounds like a hell of alot in my (admittedly limited) experience of balancing Mi pistons, where they've generally been within 2-3 grams of each other. Are all the pistons from the same engine and all in the same state (ie rings removed, no gudgen pin circlips etc)? Are you using a set of scales that's accurate and (more to the point) repeatable? Share this post Link to post Share on other sites
benlilly 0 Posted October 30, 2006 Hi all, I was expecting them to be closer than they were and I have double checked the results. 2g - 3g would be much easier to deal with. I weighed them fully stripped and cleaned and they are all from the same eingine. Another Ben from Stroud on the forum. Ace! Ben Share this post Link to post Share on other sites
PumaRacing 2 Posted October 30, 2006 Hi, I weighed my pistons last week and found there to be between 4 and 9 grams difference between them. After weighing a slice of ali rod, it was pretty clear that 9 grams of metal is alot to remove from the inside rear of the piston (where I originally though to remove material) and that perhaps it was better to take the material off the flat where the gudgeon pin is located. This seems like a safe area to machine. The Gudgeon pin is 5mm back from the existing flat face and I think I should only need to take off 1-1.5mm from each side. Anyone done this and know that it is ok. Thanks, Ben 9g is a hell of a lot of metal to remove on a cast piston. 4g is bad enough to deal with. A decent set of pistons ought to be within 2 to 3 g of each other out of the box. In fact 9g probably means there's a porous casting somewhere because if the shapes are all the same then the density must be different. I wouldn't entertain trying to find 9g if this was a customer's engine. I'd start with a better set of pistons. I suggest you remove as much as possible from inside the piston first. You can get about 4g by radiusing the corners of the pin bosses and working on the ribs which run to the piston skirts. If you have a lathe you can also machine the tooling datum, which the piston sits on when it's being made, a bit deeper. You could even shorten the skirt a tad. Just don't try and get all the weight out of one place. Share this post Link to post Share on other sites
whizzer71 0 Posted October 30, 2006 When I built my last Mi engine i used a very expensive set of digital paint scales to weigh my pistons with(brand new piston and liner sets x 4) and they were all within milligrammes of one another around .005 I seem to remember ,very pleasantly suprised at the time. Rods were a little way out but didnt take much to get them identical. Tris Share this post Link to post Share on other sites
niklas 1 Posted October 30, 2006 IIRC, according to unknown manual the maximum difference between the lightest and heaviest piston/rod-combo within one engine is 7g. Anyone can verify this? Share this post Link to post Share on other sites
benlilly 0 Posted October 31, 2006 9g is a hell of a lot of metal to remove on a cast piston. 4g is bad enough to deal with. A decent set of pistons ought to be within 2 to 3 g of each other out of the box. In fact 9g probably means there's a porous casting somewhere because if the shapes are all the same then the density must be different. I wouldn't entertain trying to find 9g if this was a customer's engine. I'd start with a better set of pistons. I suggest you remove as much as possible from inside the piston first. You can get about 4g by radiusing the corners of the pin bosses and working on the ribs which run to the piston skirts. If you have a lathe you can also machine the tooling datum, which the piston sits on when it's being made, a bit deeper. You could even shorten the skirt a tad. Just don't try and get all the weight out of one place. I agree a more closely matched set would be a much better starting point but I'm trying to keep budget down. Anyone have a good spare set going for a good price? Am I correct that the tooling datum is the flatish 'nipple' centred on the underside of the piston? Would it not be better to machine the flat surrounding the gudgeon pin rather than have pistons with different length skirts? If I did take some off the skirt, 1mm to 2mm tops? Thanks, Ben Share this post Link to post Share on other sites
PumaRacing 2 Posted October 31, 2006 I agree a more closely matched set would be a much better starting point but I'm trying to keep budget down. Anyone have a good spare set going for a good price? Am I correct that the tooling datum is the flatish 'nipple' centred on the underside of the piston? No, it's the machined register just above the bottom of the skirt. That's what the piston sits on during all manufacturing ops so the crown, ring grooves, gudgeon pin bore etc are all datumed to the same height from a known point. Would it not be better to machine the flat surrounding the gudgeon pin rather than have pistons with different length skirts? Weakening the pin boss which transfers all the loadings into the conrod ain't the best place to be lightening a piston. If I did take some off the skirt, 1mm to 2mm tops? A couple of mm isn't going to hurt anything. Share this post Link to post Share on other sites
crogthomas 0 Posted October 31, 2006 I'd weigh all the gudgeon pins as well. There is often some slight variation in most engines Ive dealt with. Although admittedly Ive not pulled a XU apart yet. Match the lightest pin to the heaviest piston and you then have less to remove from that piston. Share this post Link to post Share on other sites
benlilly 0 Posted November 1, 2006 I'd weigh all the gudgeon pins as well. There is often some slight variation in most engines Ive dealt with. Although admittedly Ive not pulled a XU apart yet. Match the lightest pin to the heaviest piston and you then have less to remove from that piston. Pins are all within 0.5g of each other. Share this post Link to post Share on other sites
