Piston Venting

jackmech · 02-12-2003, 03:06 PM

Everyone has to vent sometime, so now it's my turn.

Stopped off at my machine shop today to drop off my case for cleaning and checking. Happily, my crank balance job was done. My crank is now balanced to within 0.2 grams of perfect. This is race spec balancing. My con. rods are now reconditioned and balanced with new bushings and bearings. And my new Mahle piston and cylinder set? Bogus! They are so far outta whack that they can't be balanced (They read 491, 489, 489, 481 grams) . And the place I ordered them from doesn't know when they can get another set. So now all my momentum for building this engine has just disappeared in one afternoon. I was about to order my Webcam and lifters and was looking at getting a Mallory as well. Grrrrrrr!

Can anyone save me?! Any physics defying ideas as to where I can shave 10 grams off an aluminium piston? Guess I'll just have to face the music that it can't be done. Everyone watch your pistons. Just because they're new doesn't mean they're good.

02-12-2003, 03:27 PM

My understanding of the balancing process of pistons was to weigh all 4 pistons, take the lightest weight one (in your case 481 grams) and grind down the remaining 3 to match the weight? I agree that 10 grams diffenerence between pistons is a bit too much, but your balancer could remedy that, no?

jackmech · 02-12-2003, 03:32 PM

You're correct, but 10 grams of aluminium is a ton of metal and there just isn't much room for removal. It's been mentioned that removing metal from the piston pin might be an option. Anyone know about this? Maybe boring the center out more?

WOODPIE · 02-12-2003, 07:04 PM

I'm almost afraid to ask this, since it might be a stoooopid question. Is there any way to ADD weight to pistons that would be safe and not damage them?

Ed

Bleyseng · Bleyseng's Garage

Weld some washers on? No, just kidding. I have always brought them set back if they were that far off or taken another set to try to match up four.
Geoff

yoh!!! · yoh!!!'s Garage

Yoh! 10 gram is too much aluminum to be removed. This is also not the first time I heard someone complaining about the significant weight issues of Mahle piston kits for the 914's.
I remember that Jake R. on the Shoptalk forum mentioned that he orders something like 20 kits and them matches pistons from that lot of 80. According to him he still ends up with some pistons that are way out of the normal range, which he sends back.
Hmmm, there is an idea besides the removing material from the piston pins...
... another option might be to group the lighter pistons with the heavier rods and vice versa. Works only if you have not balanced the rods yet. And the weight difference among the rods should not be greater then +/- 2 gram anyway - so you would have to grind quiet a bit after all.
Here is my recommendation check for Kolben Schmidt P/C for the 914 - mine where within +/- 0.5 gram and the overall appearance is at least as good as the Mahle's. Order the cam and the lifters and continue on the finishing the case – That should keep you busy for a while.

MfG

Patrick

John Rogers · 02-12-2003, 09:32 PM

The pistons could have some weld buildup done to the lighter ones if the shop has the ability to heat treat them during and after welding. I would do some measuring of all the piston areas to see where there are differences and you could shave some weight there. The last set of JE pistons we got were within a couple of grams right out of the box. Good luck.

Jake Raby · 02-12-2003, 09:56 PM

10 grams in a cast piston is insane to remove!

I do buy larger numbers of piston sets and match components, but most all I use are the KB 96s and JE forged pistons. The stock piston sets are normally this far out of balance, especially those, they look like Cofap pistons. they may have a KS marking on them, but they are Cofap.

either you are going to have to balance the pistons by using the pibns, or settle for an out of balance engine, or buy another piston set.

as far as the crank goes, did you have the complete dynamic assembly balanced to include fan, flywheel and pressure plate? If you only balanced the crank, you are in for a big suprise, as the bigest out of balance components are ALWAYS the flywheel, fan and pressure plate. almost every crank I spin up will balance to tolerance the first spin......also were the crank gears installed during the process with the woodruff keys in place???? if not, yet again, you will have a suprise.

I spend alot of time on my CNC balancer, have the entire mass balanced and INDEXED for complete accurate assembly, or you will be wasting time somewhere........

Ensure the rods are balanced end to end, and not just "overall". have the engine balanced just like it will be assembled, else the balance effort is a waste....A well balanced engine can spin 8K and be smooth as silk, an unbalanced one will rip its self apart yesterday.......

Di your balance shop measure the spec in .02 grams, or .02 oz.in.??Huge difference.

maf 914 · 02-13-2003, 04:15 AM

When balancing the rotating mass what is the proper sequence? Is the crank first checked, then gears added, then fan, then flywheel, then pressure plate....balancing each item as you go? Or do you start out with the crank, gears, fan, flywheel presure plate as a unit? My real question concerns the external pieces, such as the flywheel, pressure plate, or fan. Suppose you have to replace one of these. Is your balance then lost? Or can these pieces be balanced independently before installation to maintain overall engine balance?

Mike

Jake Raby · 02-13-2003, 07:04 AM

My balancer stores the information and I can recall the data and rebalance a component.

The sequesnce is t balance every dynamic part individually, then assemble and index them together one part at a time, and correct the "Plane" each time a new part is added to the assembly.

Fotr instance I balance the crank(crank with gears and hub installed), fan, flywheel, ane pressure plate individually thenstart hanging parts on the crank one at a time, spinning them up and correcting the plane.

A balanced engine is only good if it is idexed and assembled just like it was balanced. I even stamp and index the pressure plate attaching bolts, and it does make a difference in balance. If an engine is bolted together out of index, it probably will be 30-40 grams out of balance..

So the answer to the queston is yes, I can match componenets later, either by getting the old components and spinning them up and matching a new one or by recalling the info from the data base and matching a component. I had to make mandrels for holding the TIV pieces specifically.