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#288344 - 12/11/09 5:20 am *%$^&#^* GPM Pistons!!!  
Joined: Apr 2004
Posts: 94
Wheatdog Offline
BritBike Forum member
Wheatdog  Offline
BritBike Forum member

Joined: Apr 2004
Posts: 94
Madisonville, LA
Hi guys, been gone awhile but now that I got the 67 Lightning running again, thought I'd post. It's been 5 yrs and two total engine rebuilds (Katrina drowned the first try along with my house, cars, life etc.) and 3 top end redo's and finally it looks like the god forsaken little monster has stopped seizing. GPM pistons do not stay round when heated!! The first two times I rechecked piston/bore clearance and gently opened same a little more. The third time I was pissed and took a file to the scored areas on each side of the skirt where it meets the wrist pin boss and agressively clearanced these Italian turds. Couldn't find a set of +20 hepolites so I went with the file option. Seems the 3rd time's the charm. Now that it runs well, I don't think I want to sell. Oops, getting the evil eye from my spousal unit! Is it just my luck or is everyone having problems with GPM pistons?

P.S. You know, now that I put the bastard file to those slugs, vibration seems markedly reduced at cruising speeds. Musta lucked out and hit a sweet spot on the balance.

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#288363 - 12/11/09 9:55 am Re: *%$^&#^* GPM Pistons!!! [Re: Wheatdog]  
Joined: Sep 2006
Posts: 145
zambu Offline
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zambu  Offline
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Joined: Sep 2006
Posts: 145
Switzerland
I use GPM pistons in my A10 since engine rebuild. Did 3400 miles since, no problems at all. Even on very hot days under heavy load. I thought they do good quality pistons. Have you checked all other possible causes? (Mixture, ignition timing..)
Good luck!

#288465 - 12/11/09 8:40 pm Re: *%$^&#^* GPM Pistons!!! [Re: zambu]  
Joined: Mar 2005
Posts: 9,965
John Healy Online content
John Healy  Online Content


Joined: Mar 2005
Posts: 9,965
Boston, Massachusetts
Mr. Dog... There should not be enough heat in that area unless the bike has suffered some detonation or the rings are not seating. I think if you examine things like the under side of the piston and the wrist pin you will find discoloration from excess heat. IMHO there is only a small chance that the problem started with the pistons themselves. If the area inside the dome of the piston is as it came out of the box, and the pin is still a bright silver and not straw colored or darker, I will give it to you that the pistons were the problem. But if you have any discoloration at all you are looking at a symptom not the problem.


#288481 - 12/11/09 10:55 pm Re: *%$^&#^* GPM Pistons!!! [Re: John Healy]  
Joined: Apr 2004
Posts: 94
Wheatdog Offline
BritBike Forum member
Wheatdog  Offline
BritBike Forum member

Joined: Apr 2004
Posts: 94
Madisonville, LA
Inside of the pistons appear as new. Wrist pin as well. Actually running a bit rich still while tweaking the monoblocs I had resleeved. The ring land area and rings working fine. Compression also fine. Timing is very slightly retarded intentionally. It's not running hot. Each time it seized, it scored the pistons in an approx .25" strip down the skirt adjacent to each side of the wrist pin bosses. Looking under the piston, this is exactly where the thicker wrist pin areas meets the thin area of the skirt. Don't know why but they were distorting. Like Posted, I "file fit" that area of the piston and now am not having any problems.

#288492 - 12/11/09 11:20 pm Re: *%$^&#^* GPM Pistons!!! [Re: Wheatdog]  
Joined: Sep 2006
Posts: 398
highway Offline
BritBike Forum member
highway  Offline
BritBike Forum member

Joined: Sep 2006
Posts: 398
hamilton on. can.
I've fit about a dozen sets in A65s and never had a problem. The score marks, were they the same on each side of the piston,OR siezing up at the top and streaking down on one side, and siezing near the bottom and streaking on the other?

#288497 - 12/12/09 12:18 am Re: *%$^&#^* GPM Pistons!!! [Re: highway]  
Joined: Aug 2001
Posts: 1,182
Mike Carter R.I.P. Offline
In Remembrance
Mike Carter R.I.P.  Offline
In Remembrance

Joined: Aug 2001
Posts: 1,182
Altoona, Florida
I have yet to have one sieze up on any kind of pistom....What clearance are you doing..I use .005 unless piston manufacturer gives me a spec..Nooo problems none of the bikes use oil, all is good.. I have a good shop that does the boring for me and They always check for proper clearance at skirt bottom..Just a thought..

Sorry about your luck I love my 67, great bike.

Mike Carter

#288607 - 12/12/09 7:50 pm Re: *%$^&#^* GPM Pistons!!! [Re: Mike Carter R.I.P.]  
Joined: May 2004
Posts: 3,547
BSA_WM20 Offline
BritBike Forum member
BSA_WM20  Offline
BritBike Forum member

Joined: May 2004
Posts: 3,547
Sydney Australia
If you really need to verify you can always pop the pistons in the oven, bring it up to full heat, leave them in there for 15 minutes and then whip them out and run the veniers over the problem area.

Metallurgist found out about hydrogen inclusion ( and thus piston growth ) during WW II . Since that time foundries have been degassing aluminium prior to casting.

Most foundries will take several test pins and check them for inclusion during he pour cause it is cheaper to toss them back in the furnace than to machine them then have to do a recall.

Having said that I used to see thousands of pistons coming into the scrap yard and it was very tempting to "liberate" some that looked "good". So if they came from evilbay there is a good chance that they were scrap before you bought them.


Bike Beesa
Trevor
#288839 - 12/13/09 10:36 pm Re: *%$^&#^* GPM Pistons!!! [Re: BSA_WM20]  
Joined: Apr 2006
Posts: 1,395
johnm Online content
BritBike Forum member
johnm  Online Content
BritBike Forum member

Joined: Apr 2006
Posts: 1,395
New Zealand
I have used high comp GPM pistons (10.25:1) in my 500 Norton twin for the past 7 years for Classic racing. Sorry I cannot remember the clearance I used.

No issues but I will replace them shortly because they have been in there too long for racing.

Piston speeds are not too high however with the 72.6 stroke and limited to 7200 rpm. You have to go over 8000 rpm to reach to the old rule of thumb of 4000 ft per min.

The biggest issue with GPM is that my friends in the business tell me they are crazy to deal with and they never know what will turn up when they order something. You cannot buy the GPM pistons I used now and I have had two sets of Aries pistons made for replacements.

#288852 - 12/14/09 12:19 am Re: *%$^&#^* GPM Pistons!!! [Re: johnm]  
Joined: Apr 2004
Posts: 94
Wheatdog Offline
BritBike Forum member
Wheatdog  Offline
BritBike Forum member

Joined: Apr 2004
Posts: 94
Madisonville, LA
Some of my tendancy to overlook important things showing here. I'm embarrassed. Never did check actual clearances after delivering pistons and barrel to "so-called" Brit Bike experienced shop for +20 rebore. It may well be they were always too tight and my problem was insufficient clearance, not the pistons. Doh!! Anyway, its running fine now with no tendancy to grab. I'll put a couple of thousand miles on it to make sure it's OK then sell. It's a numbers matching 67 Lightning with almost all the correct parts including the original monoblocs. I've got a spare T-Bolt engine that runs great and access to all the parts needed to make another bike. I'd rather run a nice mismatch parts bike and bank the cash from the more collectable '67.


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