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MG MGA - Piston dimensions

I just bought a set of Aerolite +040 pistons for a Mk2
Checking them for diameter I found (typical of the 4)
Lateral: top 3.0354, skirt 3.0372; In line: top 3.0232, skirt 3.0238. So quite a bit of consisten ovality and taper. I thought pistons were turned on a lathe so must be round. The instructions recommend a clearance of 2.5 to 3 thousandths (presumably at the largest dimension.
My question is, is this normal? If it is, then why? The old pistons don't show this. Will the machinist be confused?


Also, the crank thrust washers packet says"Nissan/Datsun"! What was the connection there with BMC?
Art
A Pearse

Art,

Pistons do have ovality and taper. They are not turned in a lathe to be circular, but are CNC machined to the correct oval profile. This is because they do not expand evenly when heated. This is because they not symetrical due to the need for extra metal for the wrist pin (gudgeon pin).

Datsun used a version of the BMC engine.

Mick
M F Anderson

Thanks Mick. After Googling "piston ovality" it seems it is modern practice to make them fit tighter across the gudgeon pin axis and to taper from the skirt to the top. But the explanation of different thermal expansion for ovality does not seem logical. In my case there is about 12 thou difference which implies a 140 deg C difference in average temperature across the piston crown, which would mean front and back have to be about 280 C hotter than the average of across the piston. The pin bosses would tend to draw heat away from these areas I would think. So I think the reason is mechanical, to stop the piston rocking; fore and aft can be relieved without affecting this (pin stops rocking in this direction). Presumably the original pistons were circular, before they made fancy machinery to make them otherwise? Maybe the exhaust valve heats up one side but then the intake cools the opposite.
A Pearse

Art,

It is not related to different temperatures at different points on the piston. The piston needs to be oval when cold, even if the temperature is same all over the piston when hot.
Because of the assymetrical construction, seen when looked at two vertical cross-sections, at 90 degrees to each other, the piston does not expand evenly.
It is oval when cold and circular when hot, even when the heat is evenly applied.
See image.


Mick


M F Anderson

Mick, surely an assymmetrical object expands the same fractional amount in all directions as it warms up?
Art
A Pearse

You all must remember that the piston skirt is on the perpendicular axis of the wrist (gudgeon) pin. During operation, this is a friction point on the piston, hence, a higher heat buildup in that area.
mike parker

Different thiknesses of the same material expand and contract at different rates. It is not as you assume Al. It is worse in air cooled engines. An aircraft engine must be preheated when below freezing. If they get too cold they can loose all clearance and bind up. Trying to start in that condition can ruin an engine.
R J Brown

RJ, that is the first I heard of that theory. So, do larger pieces expand/contract more or less ? And why?
Art
A Pearse

Art, the key here is the rate, i.e. speed of heating and cooling. The pistons are being heated by combustion (and perhaps friction) and cooled, primarily by the oil. These actions are taking place simultaneously on different surfaces. Consequently, even if equlibrium is reached (continuous even running at stable engine temperature), different parts of the piston will be at different temperatures.

Neil McGurk

Here is how I view it. I may be wrong, but it makes sense to me.

Assume equilibrium....

The top surface of the piston will expand from the center - outwards at a linear rate (CTE) based on the material (alloy). That means the diameter increases linearly but at 3.14 times the rate the circumference does(circumference = Pi*d)

The shape of the piston at the bottom of the skirt is more like a tube. The circumference increases at the material's CTE. The diameter increases at a rate of 1/pi * CTE. almost 1/3rd slower than the above example. This is due to the geometries of the parts.

In bot examples, both the top of the piston and the bottom of the piston's skirt stay round as the temperature increases.

Now, in the middle of the piston, where the wrist pin (gudgeon pin) is, there is a boss of aluminum added internally to support the wrist pin. This is only in one direction, front to back of the piston.

Considering these geometries, the shape of the piston at the centerline of the wrist pin will now change as it heats up because; 1) the diameter will increase (slowly)based on the above example of the bottom of the skirt; and 2) it will furter expand only along the axis of the wrist pin 3) at 90 degrees from the axis, the piston will not further expand.

So, at the top of the piston, the shape stays circular as it expands, as does the bottom of the skirt. Only at the wrist pin, does the "shape" change.

If I believe all this, then the ideal shape of the piston is round at the top. changing to somewhat oval at the wristpin and back to round at the bottm (but a bit smaller than the top. This is assmong that we want the piston to be a perfect circle at temperature.

C Schaefer

Sorry, "assmong" in that last sentence should be written "assuming". Darn fingers!
C Schaefer

Lots of good ideas! How about this one - The pin boss areas actually run a bit COOLER b/c they conduct heat away from the crown faster b/c of greater cross section. So the hottest side is at 90 deg to the pin axis. Since the piston wall in this area expands fore and aft, it would tend to make the piston longer along the pin axis (and vice-versa logic for the other direction) That is why they make the shorter diameter along the axis. I did not measure the very top of the piston, just above the pin ("top") and the bottom of the skirt.
Incidentally, in my googling I discovered that Honda made a race bike motor with true oval pistons, to make room for 8 valves per cylinder!!
A Pearse

You have to remember too that the piston is not a single alloy, but has specific alloys at specific points of the piston. While you might expect a longtitudinal expansion, for example, the effects of the different alloys and their specific placement in the item would cause a different type of expansion.
mike parker

a quick Goolge entry brought up this link
http://www.motorcycleproject.com/motorcycle/text/cows-pistons.html
M Gannon

Excellent article Mark. But I think he got the temperature thing reversed. The bosses are cooler and shrink the shape across the pin, causing the piston to elongate along the pin, and thus end up circular.
A Pearse

Mike, what piston do you know of that has different alloys in different areas? Pistons are either cast or forged. In either case it would be one homogeneous material used throughout. It would be difficult to manufacture them in multiple alloys. Maybe some high-tech stuff is out there. Or maybe I don't understand your comment. I'm confused???
C Schaefer

CS,
Maybe they're not being made that way, but I have discussed this with some engineers at one of the Big 3 companies about 3 years ago.
I understood from my discussions that quite a few of today's cars run with composite pistons, aluminum over another type of metal for the wrist pin area, or special metals on the crown, under the aluminum, for heat dissipation.
Apparently it isn't very common over on this side of the pond, but the Czechs and Russians are looking at it. There are a number of patents online for bimetallic pistons.
mike parker

Mike, I did a search on "composite piston" and learned a lot. Being an MGA nut, my piston knowledge must be stuck in the 1950's. I wasn't aware of the technology now available. Thanks for the lesson. Now I've got some studying to do!

On a second note, I think the layman's understandings offered in the above discussion in regards to piston ovality are just generalizations. Absoulte statements cannot be made until a specific piston design is modeled and FEA analysis made.

Chuck
C Schaefer

I sort of doubt I have a composite piston for the price I paid!
One thing still bothering me - the original 1961 pistons were probably round, and clearance spec was smaller than the current ovality relief, so why did early engines not seize when hot ?
Or did they make oval pistons back then?
Art
A Pearse

Art, the pistons were round. The cylinders were oval!

Just kidding! I dont know why. But consider back then it was common practice to reuse the pistons and knurled the skirts to fit the new bore. I wonder what the roundness was when they did that?
C Schaefer

Art,
Most probably they didn't have any equipment to measure temperature of a piston in situ like they do now, so never noticed the out of round. Perhaps the heavier pistons of that era did heat more evenly, while the new pistons,being engineered for lightness, with special reinforcements at specific areas under the crown, create that uneven heating that wasn't a problem before.
mike parker

This thread was discussed between 14/02/2009 and 18/02/2009

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