The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: snowbeard on 13.05. 2009 19:16

Title: damaged pistons?
Post by: snowbeard 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.
Title: Re: damaged pistons?
Post by: rocket man on 13.05. 2009 20:27
i would get a new set ove pistons and rings
Title: Re: damaged pistons?
Post by: rocket man on 13.05. 2009 20:28
also i would find some good rods i wouldent trust them
Title: Re: damaged pistons?
Post by: a10gf on 18.05. 2009 19:55
Quote
i wouldent trust them

agree, do not take any chances with this category of engine parts.
Title: Re: damaged pistons?
Post by: BSA_54A10 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. 
Title: Re: damaged pistons?
Post by: MikeN 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
Title: Re: damaged pistons?
Post by: BSA_54A10 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 )
Title: Re: damaged pistons?
Post by: MikeN 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.
Title: Re: damaged pistons?
Post by: fido 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.
Title: Re: damaged pistons?
Post by: snowbeard 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!
Title: Re: damaged pistons?
Post by: RichardL 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
Title: Re: damaged pistons?
Post by: A10Boy on 20.05. 2009 17:44
Snowbeard

They are scrap forget them! You can buy new conrods and pistons.
Title: Re: damaged pistons?
Post by: A10Boy on 20.05. 2009 18:06
LOL, yeah thats about all they are good for!

 *smile*
Title: Re: damaged pistons?
Post by: RichardL 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. 
Title: Re: damaged pistons?
Post by: A10Boy 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.
Title: Re: damaged pistons?
Post by: groily on 20.05. 2009 20:04
Now we're talking. My 2 favourite ashtrays are 1. a Rolls Royce Griffon/Griffin (whatever) out of a late-marque Spitfire, and 2. a 4.2 Jaguar piston. For lighter addicts - pardon the pun - I recommend the pre-war Austin 7: flat top and no wobble. Just have to empty it more often. There's always a rôle for a discarded piston . . . and Snowbeard's would make a nice addition to the life-shortening array round here. Not that there aren't a few waiting for the snip . . . .
Title: Re: damaged pistons?
Post by: fido on 20.05. 2009 20:21
I remember some years ago when the old boy who ran C&D Autos was getting too ill to work he suggested to his wife that she could carry on after he died, just selling pistons and not bothering with the other bike bits. Apparently he had acquired a huge number of pistons and I've often wondered if they still have them, now the sons run the shop.
Title: Re: damaged pistons?
Post by: MikeN on 20.05. 2009 20:50
manosound,
  Yes you are correct,the 6" or longer test mandrels are to give an exagerrated error. The piston pin isnt really long enough to detect the error you would be looking for.
   Also,you need a good small end bush already sized for your piston pin before you can start checking with mandrels .
 There is a great old book written by Phil irving  (designer of the vincent twins?) called "tuning for speed" .It tells you how to build an engine properly and check and rectify errors and wear.They often turn up on ebay (but  can be a bit pricey). It was written in the 50's but it is full of good stuff.it explains all of the above
 Also regarding the comment by Fido, the rods supplied to me by SRM dont look to me like they have been bodged up in a shed ,they look like they have been beautifully CNC machined from large lumps of high quality alloy.
  Mike
Title: Re: damaged pistons?
Post by: a10gf on 20.05. 2009 20:53
I got an ermail from this company, maybe of interest

http://www.vintagemotorcyclepistons.co.uk
Title: Re: damaged pistons?
Post by: BSA_54A10 on 21.05. 2009 01:09
As stated previously, clean them up , measure them and crack test them.
If they are in good nick then reuse them.
If they are a little worn then you can get away with a set of Total Seal gapless rings.
Critical measurement is the ring gap. If it is too big then they are remelt material.
If they are +20 then they can be machined back to Std so tag them and stick them in the box till you bump into some one with a thirst and a lathe (std +20 & +40 ) are usually machined from the same blank which is why most engines only have 2 over sizes +60 +80 are generally machined from a bigger blank which is why they are not offered very often.
Title: Re: damaged pistons?
Post by: coater87 on 21.05. 2009 04:27
I got an ermail from this company, maybe of interest

http://www.vintagemotorcyclepistons.co.uk

 Anyone know why these guys dont offer .10 over pistons?

 Lee
Title: Re: damaged pistons?
Post by: BSA_54A10 on 21.05. 2009 05:24
Well I checked out a couple of different models , just to see what was there.
M20 pistons are GPM and they only have 2 of them in stock
B34 were the same but only 1 in stock.

Best guess they have bought out some ones stock then done some clever HTML work so they look like a massive concern.
There are very few std or first oversize pistons listed for anything and no photos of stock, just 1/3 of the description that you would get if you were browsing an aftermarket piston makers catalogue.

Reciently I was looking for some round wire internal c clips.
Found about 7 different catalogues on line with extensive range but when I tried to do an order, no one actually had any in stock and all bar 1 of them had never stocked plain round wire circlips but the computer geek that did their web page had just coded in the entire catalogue of each of their suppliers so their sites have an impressive 300 pages of stock most of which they did not have and never ever stocked and will never stock.
I think I mentioned this in a previous post.
YOU CAN NOT BELIEVE ANYTHING THAT YOU READ ON THE WEB UNLESS YOU HAVE INDEPENDENTLY VERIFIED IT IS TRUE.
Any supplier that give you a turn around of anything more than 1 day has no stock and is just playing middle man between you and a wholesaler somewhere oft in China,  India, Indonesia or the Phillapines.
Two day for him to get it from them then 2 more to get it to you. 
Title: Re: damaged pistons?
Post by: fido on 21.05. 2009 08:11
manosound,
 
 Also regarding the comment by Fido, the rods supplied to me by SRM dont look to me like they have been bodged up in a shed ,they look like they have been beautifully CNC machined from large lumps of high quality alloy.
  Mike

I don't know how the BSA ones were made but they were certainly not machined from stock material. As you know, a forging would be stronger as the grainflow follows the shape of the object. Having said that, if good original parts are not available, SRM products would be my next best choice.
Title: Re: damaged pistons?
Post by: RichardL on 21.05. 2009 12:10
I wonder, for the SRM rods,

...is the billet forged (hot or cold?) to a very high strength rectangular shape, then machined?

...is the rod forged to near its final shape then cleaned up by machining?

...is the final shape after forging so good that it does not need machining other than big end, small end and bolt holes?

One thing I am rather certain of, you can't just go to your local metals yard and buy a block of 6061-T6, machine it yourself, and end up with the same product. Did I mention that these rods do not require small-end bushes?

Richard L.



 
Title: Re: damaged pistons?
Post by: A10Boy on 21.05. 2009 16:41
Just going back to Fido's comments on C&D Autos, I was in there today getting some fork stantions, bushes etc etc, and I asked him about the pistons. Apparently they sold them all years ago and dont have any stock at the moment but can supply some eyetie ones.
Title: Re: damaged pistons?
Post by: A10Boy on 28.05. 2009 13:27
Its seems that Wiseco are making "Rocket" pistons and these are available in the uk via cake street classics. Not sure about other compression ratios - does any one know ?
Title: Re: damaged pistons?
Post by: beezalex on 28.05. 2009 14:51
Richard, here's an excerpt from MAP's catalog (they make SRM's rods):
Quote
M.A.P. Rods are Strong as Steel with LESS Weight. Compare our FEATURES to the Cost! You'll
understand why M.A.P. makes the BEST HI-Performance Products for the Best of British Motorcycles
STANDARD FEATURES:
True FORGING: Made from Specially Selected "Ultra-Strong" 7075 Alloy FORGING
C.N.C. Machined: Exact Specifications EVERY Time.
Light Weight: POWER Increaser with Less Vibrations than steel counterparts
(290 gram Big-End with 390 gram Total Weight (prox. T120) 420gr stock: 550+ Steel)
Machine Polished: Normalize "Stress Risers" for the Ultimate in Strength.
Serrated Big-End CAP: Allows Repeat Concentricity with "Rugged" Reliability. Eliminates Cap Walk

This is confirmed by conversations with the owner who told me that they are machined from billet forged aluminum, so they are apparently machined from big forged chunks. This makes sense since the tooling for forging to finished shape would be too high for such a small-volume part.
Title: Re: damaged pistons?
Post by: RichardL on 28.05. 2009 15:39
Alex,

Thanks. I forgot to look for such info on the MAP site (I knew they made the SRM rods, from talking to SRM). This is how I thought it must be, but I got a little confused by Fido's comment, which is not wholly wrong,as they are not simply machined from some random lump of melted down bear cans.

Richard