Author Topic: damaged pistons?  (Read 3830 times)

Offline snowbeard

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damaged pistons?
« on: 13.05. 2009 19:16 »
sorry to just bog down the server with all my questions, but upon inspection of my parts, the con rod and piston set has experienced a seizure at some point.

one rod is obviously bent, and the piston skirt has contacted the flywheel afterwards.  it has a small wear mark on the  inside edge, I'm assuming the person got it unstuck and tried to run it again anyway.

you can kind of see the bent one, and also how much blowby they had in this pic.

http://img.photobucket.com/albums/v632/snowbeard/conrods.jpg

so my question is, how bad is this, and what should I check on if I wanted to put them back into service?

I've got them soaking in Berryman's right now, so I can get better pics of them clean later on, as well as a better view of any issues.
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Offline rocket man

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Re: damaged pistons?
« Reply #1 on: 13.05. 2009 20:27 »
i would get a new set ove pistons and rings
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Offline rocket man

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Re: damaged pistons?
« Reply #2 on: 13.05. 2009 20:28 »
also i would find some good rods i wouldent trust them
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Offline a10gf

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Re: damaged pistons?
« Reply #3 on: 18.05. 2009 19:55 »
Quote
i wouldent trust them

agree, do not take any chances with this category of engine parts.
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Offline BSA_54A10

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Re: damaged pistons?
« Reply #4 on: 19.05. 2009 01:47 »
Usually I would also dump them in the "some day I might need them box" and buy some new ones.
But in any case they need to be cleaned and inspected to decide if they go into that box or the "melt down" box.
Start with a wash in degreaser, then carb cleaner, then a soda blast.
From there it is out with the micrometer to check for wear and then if they measure up then grab a Dye Penetrant Test kit, Turco market them worldwide in 3 spray cans.
If they are good put them in the someday box if they are bad smack them wit a hammer so that it is obvious that they are bad which prevents them ending up on ebay in 20 years time. 
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Offline MikeN

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Re: damaged pistons?
« Reply #5 on: 20.05. 2009 10:15 »
When you find your new con-rods ,they may look fine but be sure to check them or have them checked for alignment if they are from a used bike as they are very often misaligned (slightly bent). I have built up few alloy con rod engines aver the years (Triumphs and BSA's) and One thing I always check is the rod alignment .I expect them to be bent and I am not usually dissapointed. I am sure its caused by careless owners  knocking out the gudgeon pins (wrist pins?) without warming up the piston to free it up.Because they are often both bent inwards.
  To do this you will require some slightly specialised equipment. Surface plate ,DTI guage and base ,
A pair of vee blocks and close fitting mandrels for both ends at least 6" long .If you are handy with a lathe you can make the mandrels .
  You need to fit the mandrels to the rod and stand it upright in the vee blocks and use the DTI to detect bend in the rod .You must then correct it ifyou still wish to use the rod.Then you must lay the rod horizontal and check and rectify for twist.I like to see better than .003" over the 6".
   Using bent rods will reduce the life of your engine because they put all the bearings and pistons out of alignment and cause excess friction and wear and piston ring blow-by.Im not talking about a lot of bend here.You probably wont detect it with the eye.If you can then they are probably past saving.
  By the way,the giveaway for misaligned rods on an engine that you may be working on is that you will see scuff marks on the top land of the piston at (say) the top Left hand side and at the bottom r/h side.
  Hope this is of interest .Mike
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Offline BSA_54A10

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Re: damaged pistons?
« Reply #6 on: 20.05. 2009 11:31 »
Just to confuse the matter even more barrels that have been bored out of square will also cause the exact same markings on a piston and barrels bored at a very slight V or offset front back are not as uncommon as one would hope.
Then again the last really good cylinder shop in Sydney ( Master Rebores ) closed down last year because they charged a fair price $ 60.00 to do the job properly so most riders took them to the $ 20.00 a side cowboys ( and got bowlegged bores )
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Offline MikeN

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Re: damaged pistons?
« Reply #7 on: 20.05. 2009 11:57 »
Also,Looking at your picture  its not clear to me which is the damaged c/rod as neither of them look too clever. I am refering to the dings and gouges up the lentgth of the "I" section part. Any dent damage on a rod will  be a stress riser and can lead to the start of a fracture which would not be good.
 When I used to have my Triumph ,whenever I went above 70mph I used to get this vision in my mind of alloy con-rods flashing up and down very fast. So when i built my A10 i asked myself  "do I really want 55 year old rods in my engine" and the answer was no ,so I splashed out and bought new ones from SRM.
  it would be interesting to hear from someone with specialist metalurigical knowledge what the affect of age is on alloy rods and other components.
 in fact,i think I will start a new post on this subject.
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Offline fido

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Re: damaged pistons?
« Reply #8 on: 20.05. 2009 13:36 »
I would rather put my trust in rods made by BSA 50 odd years ago than something bodged up in the shed of a pattern parts maker. Obviously any surface damage should be carefully polished out and I would agree with the others that straightening a bent conrod and re-using it is quite a dodgy practice.
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Offline snowbeard

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Re: damaged pistons?
« Reply #9 on: 20.05. 2009 16:49 »
the darker rod sort of on it's side is the bent one, and it is noticeable by eye.  I don't plan on using the rods at all, I was more interested in just the pistons.

  I have rods on the both possible engines that I "trust" to some degree, new wellworthy .040 rings, and what appears to be a nice .040 barrel set (I should take that to the shop for verification...)

I just put in a bid on fleabay for new pistons, so we'll see what a nice overbore set is worth to the world. ;)

thanks for all the advice, especially the info on how to check rods.  I wish I had a lathe, but just to confirm, you want the mandrels long enough to exaggerate the bend in the rod, correct?  an unnoticeable bend at the head will be more measurable  six inches out, right?

Thanks again!
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Offline RichardL

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Re: damaged pistons?
« Reply #10 on: 20.05. 2009 17:18 »
Snowbeard,

Following on Fido's comment "Obviously any surface damage should be carefully polished..." I thought you might be interested in these two videos. The first is before someone here (I think, dpaddock, but I'm not going back to check) said I should polish out the nicks, the second, after I did it. Bear in mind that even after I say in the second video that I'm happy with the wobble correction, I wasn't, and ended up getting new billet rods as shown amongst my videos as "...corrected redux". I know you are not planning on using the rods shown, but this might apply, regardless.

Richard L. 

http://www.youtube.com/watch?v=5IYligiWUac
http://www.youtube.com/watch?v=A_t5BODxGk8
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Offline A10Boy

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Re: damaged pistons?
« Reply #11 on: 20.05. 2009 17:44 »
Snowbeard

They are scrap forget them! You can buy new conrods and pistons.
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Offline A10Boy

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Re: damaged pistons?
« Reply #12 on: 20.05. 2009 18:06 »
LOL, yeah thats about all they are good for!

 *smile*
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Offline RichardL

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Re: damaged pistons?
« Reply #13 on: 20.05. 2009 19:00 »
Nevertheless, for me, it's a bit difficult to tell if the pistons are completely beyond saving. Pistons do seem to be one of the scarcer things to find. So, tossing them out without careful consideration might be a bit premature. Given a choice, go new. When the parts apocalypse comes, you may be happy to have these. And don't forget, if they don't work out, paper weights is another option.

Richard L. 
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Offline A10Boy

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Re: damaged pistons?
« Reply #14 on: 20.05. 2009 20:00 »
Yes, to be serious, I would take the pistons off the con rods, they are definitely scrap, too many dings and dents. and see what the pistons are like when cleaned up - but they do look in very bad shape.
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