lotustwincam
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| posted on 30/8/07 at 08:17 PM |
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New Pistons - weight difference
Finally got my modified replacement pistons from a local engine machining firm.
I broke the ring lands on two pistons in my crossflow a few weeks back (suspect caused by detonation due to running too lean).
Anyway, in the end I decided to replace just the two damaged pistons. In retrospect this was a BIG mistake.
A local firm eventually sourced the replacement pistons and machined them to match the cut outs of the originals.
I picked them up today, and the machining is a reasonable job. Although both old and new pistons are made by AE, I noticed when I got them home that
there is considerable differences around the gudgeon pin area.
Popping them on the kitchen scales, there is a weight difference of about 35grams!!!! The new ones being lighter.
Would it be acceptable to use the new lighter pistons on opposite crank throws to the original pistons. So for example front to back, Light, Heavy,
Light Heavy. Something like this
Or like this
Or should I try removing some weight from the old pistons? The thing is, 35g is a lot of weight to loose. What region would I try to remove it from
without risk of weakening the pistons?
Or should I just use the new pistons as expensive ashtrays, and get myself a matched set of four? According to Vulcan, they (1300 +0.060) won't
be available again until end of September.
Pistons broke and pissed off Drew
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ned
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| posted on 30/8/07 at 09:10 PM |
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If it were me I'd machine a bit off the bottom of the skirt of the piston. If you take it off in a lathe it shouldn't affect the balance
too much as will be removing equal amounts all the way round and hopefully won't have to take too much off as removing it from the entire
circumference the weight should add up quickly. A friend weighed his pinto pistons and got he weights near the lightest using this method. He balanced
them to with 1 or 2 gramms on a set of little digi kitchen scales.
Ned.
beware, I've got yellow skin
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jambojeef
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| posted on 30/8/07 at 09:23 PM |
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If I were you I wouldnt touch them!
Ned could be absolutely right that machining the skirts could achieve balance of the pistons but if it were me Id assemble it and if you can send the
flywheel, crank, con rods and pistons off to be balanced.
Even if you get the pistons balanced to within an acceptable range of each other the crank counterweights may not be equally acceptable and so I
reckon you're better off getting the entire rotating assembly balanced as one piece.
Hope thats helpful
Geoff
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lotustwincam
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| posted on 30/8/07 at 10:13 PM |
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Thanks for the advice.
Ned, the problem with machining the bottom off the skirts, is that the skirt on the Xflow is cut away around the gudgeon pin area.
To give you an idea of the weight that needs to be lost, a 50 pence piece weighs 8 grams. so I need to lose 4 1/2 of them or £2.25 worth!
A £2 coin weighs 12g, so I need to lose three of them!
Either way, thats a lot of metal.
Having said that. To put it into perspective, the total weight of an assembled piston is almost 600g (IIRC).
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Fatgadget
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| posted on 30/8/07 at 10:39 PM |
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Lets face it. A crossflow is an agricultural engine that was designed and manufactured back in the dark ages where tolerances were measured in
pounds and inches! ...I bet you if you put it together as is,you won't notice a blind bit of difference!
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lotustwincam
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| posted on 30/8/07 at 10:47 PM |
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Fatgadget, thats the sort of answer I was wanting to hear. :-)
I do hope you're right!
The last thing I want is to hit 6500rpm and for the engine to shake itself apart! Or worse still, to do so long before that.
I have been known to rev it to 7000+ in the past. I suppose those days are def over.
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DarrenW
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| posted on 31/8/07 at 08:33 AM |
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Is it possible to source 2 new pistons so you have a full set of matching?
Personally something doesnt sound right with your proposal, and as you know in engineering if something doesnt look or sound right then often it isnt.
Im no engine expert but my logic says it would be far worse to put it all back together to find it was wrong later and have more hassle and cost to
put it right. For piece of mind id opt for getting the weight the same or 2 more pistons. I have to say however i have no actual experience of engine
building / pistons - just a few years engineering experience.
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MikeRJ
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| posted on 31/8/07 at 09:13 AM |
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35 grams does seem a large difference. How much do the pistons actualy weigh? (quite heavy as it's an xflow!).
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tks
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| posted on 31/8/07 at 12:45 PM |
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on a bike engine the piston is also in the gramm region of tolerance.
i would try to sell the 2 not machined pistons and ask the company for another 2 machined pistons, then you should be close to the other 2 pistons
haveing a set of 4.
Tks
The above comments are always meant to be from the above persons perspective.
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Angel Acevedo
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| posted on 31/8/07 at 09:15 PM |
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quote:
Originally posted by lotustwincam
Would it be acceptable to use the new lighter pistons on opposite crank throws to the original pistons. So for example front to back, Light, Heavy,
Light Heavy. Something like this
Or like this
I`d assemble as in 2.
Beware of what you wish.. for it may come true....
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