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Seeking advice on where to remove material when balancing pistons

Practical Engine Building

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I know this was covered briefly in the lesson, but I need to get almost 3 grams off one of these pistons. They are 30 year old Mahle pistons out of a VW engine. I had to get 3 grams off the rods, which seemed like a lot of material to remove even though the rods are steel. Removing an even larger volume of material seems risky, and I'd like some advice on where I should be grinding away at these things.

They appear to be cast aluminum with strips of steel reinforcement in the casting.

I have my own ideas of course, but my goal is objective opinions of professionals. Thanks HPA crew for for finding a real void to fill in the education market. I've really been loving it.

I'd be very surprised if you'll get 3 grams out of that piston without affecting the strength sorry. Due to the density of aluminium we understandably need to remove quite a lot of material to make a dent in an imbalance. Personally I'd be inclined to find a friendly machinist and remove material by turning the piston in a lathe and deepening the step under the skirt. If you retain the existing skirt thickness and the machinist leaves a nice smooth radius to the cut then it shouldn't have any negative impact on strength. There's still a limit to how much deeper you could take this before you start reducing the strength of the pin boss though.

I got the rods for this build perfecty balanced on both ends. Do you think it's bad if I leave these pistons as-is and just go with it? I wanted my rainbows and unicorns on this one, but I also need to get this thing back together without doing a butcher job.

i would be making a good attempt to get it close if possible

Regards Ross

I still haven't gone forward with this, mostly because of developing and tracking my other summer fun car all season. The real reason though I haven't put this engine together on the side is this whole piston balancing question. It's equally as expensive to have a machinist lathe them down (hoping they are as precise and I am) as to get new aftermarket ones ($600-$800usd). These came out of a working motor that came to me after the headgasket was blown and the car was rough otherwise and undriveable by any sane standards. They clearly had at least 10 years (if not 15-20) in service without unusual wear/damage by the time it came to me, but I've never driven the engine for more than 2 minutes around a parking lot. It seemed normal, but this car will see limited street/track duty and I want it to be smooth and solid.

I have no experience driving anything I know to be balanced or imbalanced. I want to make this my own OE+ rehab of this engine, but I'm looking for professional advice on this. I can afford whatever is needed, so the question is:

Should I throw in the towel on these pistons?

Hi Mike,

from the images above they look a good quality casting.

I agreed with Andre, removing that kind of weight from a Ali piston is pushing it. Have you weighed the assembly? (Piston/rings/pins/bearings/bolts) as if these balance then I wouldn’t worry about the pistons being perfectly matched.

If you can weigh the lot and let’s see what the results are?

What options have you looked at for new pistons? Things have moved on with technology and skirt coatings and materials etc

When an engine is balanced it’s going to produce less nasty vibrations/harmonics; over an identical engine with no balancing and lightening etc the revs will be smoother and faster ramping but also faster to drop (if lightened flywheel/clutch assembly is used)

hope this helps to answer some of your questions, post back with the weights if possible.

In the past on superbike engines I’ve taken material away from the piston skirt opposite the thrust face with success but they were a more developed piston design.

It looks like your pistons rely on hoop stress for strength in the skirt as there’s no box bridge to support the pin boss so if you do end up taking material away you’ll need to make sure it’s even around the skirt (oil squirter relief not withstanding) as Andre suggested.

As Dave mentions weigh the Piston pins etc, you might find they have a similar opposite difference that makes the actual total piston weight closer than 3 grams.... certainly I had small weight differences in the pins on the pistons I balanced recently.

I just thought of something so simple for this. Is there any reason It wouldn't work to grind 3 grams of material out of the inside bore of a pin? The 20mm pins are 96.7-97.1 grams . With these pistons/pins probably never even seeing 50hp per cylinder, it seems reasonable to slightly weaken a part as strong as the pin. Thoughts?

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