pistons of varying weight

[Forktail]

Re: pistons of varying weight

Well, I'm a retired engineer...but I've been wrong before .<br /><br />My guess is that the three standard pistons were installed as a matched set all weighing the same. They probably weighed the same as the other three that were replaced too. And when the three .030 oversized pistons were purchased, they too came as a matched set. They just didn't match the others since there was only three. Usually a matched set would've included a total of 6 pistons.<br /><br />Is the weight change because of the displacement change? Tough one. I suppose it is possible since balance is affected by F=MRw^2. Here a reduction in mass would reduce the force. And an increase in angular accelleration would increase the force. More displacement usually increases compression. More compression usually raises the angular accelleration of the piston/con rod with regard to the crank (more power). So I suppose in an attempt to keep the force equal, reducing the piston weight might compensate for any added angular accelleration.

 

[OBJ]

Re: pistons of varying weight

I've been reading this thread with a lot of intrest. Fact...I printed it out so I could read it at work in my spare time..what there is of it. My regular job is a control room super in a power plant and I'm also a licensed stationary steam enginer with the state of Ohio. My shop is my retirement very soon. All my book work I've done has been 95% steam driven. X #lbs. of steam against a piston with an area of X gives you anticipated thrust X and so on. I would really like to learn more about the workings of internal combustion other than fuel/air mix in here add spark and go. I mean you guys really got my grey cells working. Is there a publication that I could get hold of that would give me the low down on what's happening in a two stroke and all the formulas that's been thrown around in this thread? And balancing. I never really understood what was going on there. I would really be grateful if someone could steer me in the right direction.

 

[dick]

Re: pistons of varying weight

Also should have said all these pistons are new and selected at random of the shelve.

 

[clanton]

Re: pistons of varying weight

The rods were changed in the 1993 V6 90 degree looper, supposed to be stronger and lighter. Pistons were new design that is 30 percent lighter, will not fit older models. Cylinder 5 and 6 has staggered compression. Combustion chamber sizes and shape are matched to their cylinder scavenging characteristics. About 4 pages of changes that were made 1993 V6 90 degree looper.

 

[Waterbugtoo]

Re: pistons of varying weight

Good conversation going on here. <br /><br />A bottom line to be noted is that on a basic stock motor, cranking lower revs, weight differences are not going to have a substantial effect. If you're building a race motor, things better be right cranking 10 grand. However, rebuilds replacing one piston IS a gamble. <br /><br />The average person of today's world absolutely DEMANDS correctness when something is obtained from another person/business, but yet, when they do something for themselves, opt for the cheapest route or advise when repairing/purchasing something. It just doesnt add up.<br /><br />The point of my previous post was simply..<br /><br />No one considering themselves or looked upon as an expert, should ever answer with a simple "yes you can", without spelling out in black and white of the risk involved. If you do, your status of "expert" is severely threatened. If a person is absolutely going to cut corners, then so be it, but to encourage such practice without thoroughly describing the risk/s is flat wrong.

 

[Dhadley]

Re: pistons of varying weight

Clanton - the "light" rods will retro. As a matter of fact they showed up in the V4 before they actually showed up in the closed deck V6. I put the light rods in my V4 looper with the 93 V6 finger ported pistons even though my V4 was not finger ported. I couldnt believe how smooth it idled.<br /><br />We did learn the hard way that the "light" rods would come apart at 9500 in a V4 on an circle race boat. We had to go back to the "heavy" rods to get past 9500. They seem to be OK in a drag motor. It has to do with the hard on-off-on thing in circle racing.<br /><br />The 3.3 rods will retro too but they are different in length by .500" so you have to move the wrist pin location in the piston. Or use the 3.3 piston.....<br /><br />Hummm.............

 

[sparkroost]

Re: pistons of varying weight

I got to reading this and found it very interesting. I had to face the same question with replacing 2 pistons or 4. I had decided on only replacing 2 pistons... until... I saw some cracks in the skirts. 30 year old cast pistons seem to get weak. I was thinking to myself if I had used them and what would have happened cause I wanted to save a mere 250.00. When I compared the weisco piston to the old cast one, there is no comparison in the rigity of the weisco and the OEM cast piston.<br /><br />2nd thing.. I had the boring done .020 over on the 2 cylinders and the other 2 cylinders were "cleaned up". Upon inspection of the STD. cylinders I noticed that the machine shops "tool" didn't touch all the cylinder, especially near the ports. There was a ridge at the top of the cylinder and a few vertical scratches.<br /><br />If it's old, replace it when it comes to pistons.

 

[Dhadley]

Re: pistons of varying weight

I'll bet if you measure the "cleaned up" cylinders you will find its at or past the wear limits. <br /><br />You might want to give some thoughts to boring those.<br /><br />Good luck!

 

[sparkroost]

Re: pistons of varying weight

DH, I dropped it off today on the way to work. I couldn't live with putting std. new pistons in there without a re-bore, considering also that the hole was NOT round. This bring into question the quality of workmanship of the machinist. I would have thought I would have been given sound advice to re-bore the other 2 cylinders. <br /> I think some hours down the road things would have crept up on me. Like 5% variation in compression or more, poor idle, who knows. At least now I have 4 new pistons .020. all new upper wrist pin bearings(which I hate the new style) new lower bearing and housing. Center bearing was cherry as well as the lower rod bearings.