The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: conordangel on 29.06. 2016 13:07
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G'day all,
I've stripped the head and barrels off dads Golden Flash and found a mildly scored bore and a piston that got hot enough to weld the rings into their grooves! I'm trying to identify this piston, to order a replacement pair. I can't for the life of me find this part number anywhere. Does anyone recognize this piston?
Cheers!
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Welcome to the forum. Can we get an introduction, perhaps?
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Hi,
This might not be as simple as just a new set of pistons.
It is going to depend on cylinder wall condition, and current size of said cylinder bores.
I dont know how many engines you have done, or your experience level at this kind of thing. But the cylinder walls wear larger over the course of miles. Your walls are also scored, so depending upon bore size and damage, you might be able to get by with a hone and a new set of pistons.
If they are in tough shape, might have to have cylinders bored over-sized and then buy new plus sized pistons to go with them.
Then, you need to figure out how or why we welded a piston tight. *conf*
More pictures, we love pictures! *smile*
Lee
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Taken the words out of my mouth , lee. pistons don't just do that theres got to be a more sinister reason I'm afraid. Whizz of the barrels and measure the bores accurately or get a machine shop to do it ,then plan accordingly, best of luck
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If it's any help, Parts list for '49-'53 shows #67-304: A10....Piston complete(up to 1951), Casting numbers are apparently one number away (I thought it went the other way *conf*)
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G'day conordangel. *welcome*.
All of the above. 67-304 is 6.5:1 compression from 1950.
Give us an intro over in http://www.a7a10.net/forum/index.php?board=13.0
Where abouts r u in this big island.
Cheers
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This is the right side piston, which is by far the worse of the two. A friend pointed out that the piston is clearly worn at four areas, evenly spaced around the piston, why is that?
I've also discovered, from the restoration paperwork (from 1988) that the bores were resleeved back to standard size. Does this mean if a hone isn't enough, we can go to the +0.020" oversize pistons? Or is re-resleeving the way to go?
Cheers!
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Having great difficulty posting photos for some reason. No idea how to shrink the file size to under 1500KB!?
Adm edit: see "Forum info & help" (http://www.a7a10.net/forum/index.php?board=15.0)
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This is the right side piston, which is by far the worse of the two. A friend pointed out that the piston is clearly worn at four areas, evenly spaced around the piston, why is that?
If the piston and bore were new and that happened, then the clearance was probably too small.
If not new, then the piston got too hot, through detonation/weak mixture/ignition timing, or labouring along at too-low rpm.
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Piston damage!
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More piston damage
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Looking like you got an interesting job :O)
Such piston damage, could starving of oil \ low oil pressure also be a cause?
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This is the right side piston, which is by far the worse of the two. A friend pointed out that the piston is clearly worn at four areas, evenly spaced around the piston, why is that?
I've also discovered, from the restoration paperwork (from 1988) that the bores were resleeved back to standard size. Does this mean if a hone isn't enough, we can go to the +0.020" oversize pistons? Or is re-resleeving the way to go?
Cheers!
Because the barrels were bored incorrectly.
To do them properly you need to clamp the barrel top & bottom and take very small cuts.
Shop used to doing car blocks usually mount them wrong then take off too much each pass and don't do enough honing.
It is called a 4 square defect, google it for the long detailed description.
You can bore A 10's out to +.080" if you are using low compression pistons. so at + 020" you have a long way to go before resleeving is needed.
First thing to do is find a workshop who knows what they are doing , cleaning up the bores then working out which oversize pistons to fit and finally finishing the barrels to suit the pistons.
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From the photos that is not overheating, it is a bad rebore done way too tight .
Unless you go to a shop that does a lot of air cooled engines most idiot moron boring machine operators bore the barrels to water cooled engine clearences and even worse a lot of them will bolt it down from the head and not from the barrel flange resulting with a bore that is too tight and not square on to the crank.
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Barrels are reported as already resleeved which I think will likely limit max oversize you can go, though +020 should surely be fine.
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It is called a 4 square defect, google it for the long detailed description.
Trevor,
I'm interested in learning more about this but having trouble locating a direct reference to "4-square defect". Found stuff about bore distortion and torque plates and sleeve interference fits, but no reference to "4-square defect". Could you steer us to a link? Thanks.
Richard L.
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You may find it if you look for 4 corner seizure.
http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=569968 (http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=569968)
The one thing that is helpful when diagnosing these kind of pistons seizures is the wrist pins.
These kinds of seizures happen because of lack of clearance. But there are two reasons it can have too little clearance:
The cylinder was finished with too little clearance for NORMAL operation. The piston did not have enough clearance for what would be considered normal piston expansion. It is normal for the heat of combustion to migrate into the piston and the resultant piston expansion is compensated for with the factory recommended piston clearance.
It is when we have abnormal combustion that we get the other reason for this kind of piston seizure. When a piston reaches normal operating temperature, and expands in the bore, there is very little extra clearance available for additional expansion. If the engine suffers prolonged detonation, which can be inaudible to the human ear, we are going to drive additional heat into the piston. If this is more heat than the engineers planned for when they specified piston clearance the piston will become "to big for the bore." We have a seizure.
So what can the wrist pin tell us. Well for one thing it was there during this event. It is also polished steel and discoloring because of it being exposed to heat will be available for us to examine. For centuries metal workers have been using the color of polished steel to tell them the temperature of the metal as it heats up. Long before the pin starts to glow red the surface will start changing color. At around 390 degrees F the bright steel color will start to turn a faint straw color, 445°F it becomes light straw, 465°F it becomes dark straw, 480°F it becomes Brown, 520°F it becomes brown/purple, 520°F it becomes purple, 540°F it becomes dark purple, 575°F it becomes blue. The thing that is handy about this is when the pin cools the color the pin reached remains there for all to see.
The bottom line to all this
If the pin comes out of the piston the same color as it was when installed - and thus only exposed to normal engine operating temperatures - the cylinder was set-up with too little clearance.
If the pin comes out displaying ANY color the engine was operating in conditions where there was abnormal temperatures and detonation is always a prime suspect in these engines! All this is especially true if there is also signs of detonation on the top of the piston or spark plug.
Why do I think the problem was detonation, not lubrication? Because lack of lubrication seizures typically start at the bottom of the skirt and go up. This seizure started just under the oil ring, a point where abnormal heat causes the most problems
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Wow I dont think I have ever seen a piston that bad before. Go to a good machinist and get him to measure the bores and if necessary hone or bore. New pistons obviously but I would definitely want to check oil flow and ignition timing before I started that motor again.