hole in piston

[lowbrowbsa]

cheers guys mg the hole was bigger on the inside but no damage to anything else valvers are good heads good boars still got hone marks i think i just got un lucky with these pistons
ive been serching and i think ill go jp 9:1
on a positive note this is all part of owning the bsa its great to ride and working on it is  as much fun 
thanks all for your imput much apreaciate it 

[MG]

This is typical for hemi engines, they have a tendency to hole their pistons when operated at high compression, even if mixture and ignition timing are set correctly and there is no detonation occuring.
Certain disturbances in the combustion process heat up a critical point on the piston crown very quickly, so the piston material becomes plastic there. Inertia forces and the fast flowing gases excavate the soft material, and material strength in this point is therefore decreasing continuously. The combustion pressure then knocks the remaining 2/3 of the material out to the inside, leaving that funnel shaped hole.
This happens in very short time, with usually no other signs of overheating on the rest of the piston crown or the skirt or signs of a seizure.

So if your ignition timing and carb are set up correctly, and there is no air leak past the head gasket, I would bet my money on this damage mechanism. Still, I'd check the head and barrel joint faces and have both skimmed if necessary!

I'm running 8.5:1 JPs in the A7 SS and they have been working properly ever since, although the bike is usually ridden like it had just been nicked to keep up with more powerful 650s.  
So if you can rule out all other possible causes, I'm sure she'll be running fine with JPs at a reasonable 9:1.

Good luck!
Markus

[Butch (cb)]

The only time I saw this was on a Daytona 500. Needed a full strip, I had alu particles everywhere inside the motor.

[RichardL]

So,  Markus,  is the crown of the left piston fractured away or melted away.  It looked fractured to me but,  as you know me,  I can convince myself of anything.

Richard L.

[MG]

We are going to bore some people to death, Richard. 

But here goes:
Hard to tell from the picture. But it looks like it started to melt on the piston top/apex, and then finally the remaining material fractured away, around the weakest spot, where most material got dragged away, or around an existing imperfection (casting defect, crack, etc.).
Could have been a totally different story of course (shot head gasket, detonation, etc.), but I thought I'd bring it up, since it could be a realistic option for what caused that failure, once all other possible issues are ruled out.
There are some arguments against detonation meltdown however. There are no signs of overheating and it happened very fast. Otherwise you would expect the piston crown to cave in, it would look burnt, you might find a blued gudgeon pin, and with a pinging engine, it will take quite some time before you finally hole the piston, if at all. With a new engine chances are rather high that it will seize due to overheating anyway. And most obviously, there will be a clear pinging noise from the engine. Furthermore, the location of the hole in the piston would be exactly opposite of the component that caused the premature ignition (ie spark plugs, injectors, glowing ridges or carbon deposits, part of the head gasket protruding into the combustion chamber, etc.). I can't see anything like that in the head, near the area of failure.
Hard to give a definite answer based on some low res pics. Grey is all theory. 

[RichardL]

Well, I didn't mean to bore anyone, including you. Thanks for providing some interesting reading. Just to torture it a bit more, I liked your point about heating up a particlar point on the piston, like a modal turbulence, as you suggest, maybe due to the hemishere. (Harder to start modes in an asymetric space.)  It seems the right piston is trying to go the same way.

Richard L.

[MG]

Not boring for me at all, failure analysis is a very fascinating thing (although rarely a nice one for the owner  ).

This is an effect well known with hemi engines, as to what the cause is, is only speculation though (and probably not very well investigated, since hemi engines are not really state-of-the-art any more, people from the USA might disagree there    ). Asymetric and roof-shaped heads are not known to be prone to this, at least I've never come across it nor read anything about such damage in relevant literature. The idea of modal turbulence is interesting though, maybe with high cylinder pressure moving the modal frequency to the "correct" levels to match the combustion process  ????

[trevinoz]

Lowbrowbsa,
                       You need to pull your engine down completely and clean all of the aluminium particles out of it.
You would be probably better off with 8:1 pistons. I went down the high compression path back in the 60s with pistons like yours. Bike pinged badly on Super of the day and would only run properly on expensive high octane fuel and then only until the barrel broke away from the flange.

  Trev.

[chaterlea25]

Hi All,
Yup!!! I have had to rebuild a couple of T140 Triumph's that did their pistons in 
The particles of alloy went everywhere, some even rolled into snowflakes by the roller bearings
and they had made their way throught the oil return system to the tank !!!!

On T140's I believe the trouble is fuel starvation usually caused by running with only one fuel tap on
ask the owners which tap they use and its the cylinder furthest away that melts down
I think it must be something to do with vibration at certain revs that causes the starvation, as there always seems to be plenty flow at a standstill ????

The BSA pistons in the pic look like 10.5's there are some on ebay to compare pics??
As Trev says 8 or so to 1 are enough for modern fuels

Oh Yea, the T140's all distorted their cylinder heads when this happened them 
So the engine internal temps must be very high before the pistons give way ????

John O R

[cus]

Holy Crap!
I ran my T140 on 1 tap for 5 years,
must have got lucky,
great reliable bike....not so good handling!

Cus

[kommando]

Think this link would be a good read, posted in response to a request for info on a holed Goldie piston.

Looks like pre ignition possibly caused by a hot spot. Could the hot spot be the exhaust valve edge in the center.

http://www.contactmagazine.com/Issue54/EngineBasics.html

[lowbrowbsa]

cheers komm thats a good read

[dpaddock]

The appearance of the piston crown is typical detonation. See these reference links:
     micromotorfactors.co.uk, and bmwvmca.org

David

[RichardL]

David,

i'm interested in reading the articlcles but the first link seems not to exist and the second is the general site for the BMW club and I can't find a reference there.

Thanks.

Richard L.

[townsends20]

hi
  I run my rgs on 10.5 to1 and after a complete re-build (still running in) I am very conscious of burning the pistons out so I set the carb to run a bit on the rich side and now using BP ultra.
you could check the points cam to make sure the gap matches on both cyl  if not it could give you an imbalance in your timing also are you running on twin carbs  if they are old it doesn?t mater if they are matching if the slides ect. are worn they will not run perfect, also air filters I think are a must that looks like a very dusty place to test all your hard work.
    Bike looks great how long did it take to build.

              Steve.  UK