3mm to 4mm Piston rings. is this Automatic to Manual??

[Dweezle]

Hey all

I recently purchased a low Kilometre Long nosed crank engine from a fellow forumite.

Upon pulling it down i have found that the Piston rings are 3mm. Not the specified 4mm.






Please excuse my CRUDE DRAWING LOL

Would this mean it is from a AUTOMATIC engine?


Now if this is the case, this engine will be down on power from the Manual wont it?

A little confused and wondering if i am jumping to conclusions here.

Any help much appreciated!!!!

[93_Clubman]

is it possible the motor's not as low km as you both thought & rings have worn?

ref auto mx5:
"The automatic transmission version of the 1.6 DOHC engine has lower compression pistons to 9.0:1 and the camshafts have less duration. These changes were done for the automatic version to gain torque at a lower rpm and minimize detonation from the torque eating trans."
http://www.solomiata.com/Mx5Engine.html

[Dweezle]

[quote="93_Clubman"]is it possible the motor's not as low km as you both thought & rings have worn?


Ummm , i suppose it is possible.. Would they all ear out the same?
I have measured half the rings, will measure the rest when i get home, See if there is any variation.
The bores are really clean with no scratches, marks etc?
Everything was in really good looking condition


I went looking for rings, and found NPR have 1.5x4mm and 1.5x3mm (auto transmission only).

I then read that same solomiata page you quoted and found out the Auto has lower comp pistons and lower duration Camshafts.....
Made me feel a little sick LOL...

[Fatty]

sounds like a good excuse to chuck in some hi comp forgies and wild cams

[Dweezle]

sounds like a good excuse to chuck in some hi comp forgies and wild cams

MMMMMMMMMMMMMMMMM.......... tempting

but this was just meant to be a clean, easy rebuild.
Then go crazy with the engine currently in the car!!

[93_Clubman]

is it possible the motor's not as low km as you both thought & rings have worn?

Ummm, i suppose it is possible... Would they all wear out the same?

ideally they would wear at about the same rate.

[Benny]

Do you know what the compression ratio is for this engine?

If you do find it is the lower compression motor and you don't want to change the pistons, you only have to have the head shaved down a little to get it back up.
If you're rebuilding an engine anyway, you should get the head skimmed anyway, so skimming it down a few extra thou won't ad much to the cost.

[Mr Morlock]

the piston rings have 2 dims on the section and you would need to check the spec as to what these are. The pistons will also have a certain ring groove dimension for compression and oil ring. The piston shape also presumably will be different for a lower comp version. You either need access to the specs or go and see one of the forum guys. Measuring bores might be an idea also to assess condition but again you might need a pro to do this for you. Its easy to spend money- if the engine is good you may well get away with little or no work to be done. If you go skimming heads you also then have to disassemble valves and get into consideration of valve seats etc etc. If the engine was good ( the one you purchased) it might have been an idea to just change it over?

[manga_blue]

Make of this whatever you can. These are all the ring sets listed in a 91-92 Mazda parts book. AT always refers to auto transmission, MT to manual.

NA6 Ring sets 1.jpg

NA6 Ring sets 2.jpg

[Dweezle]

Do you know what the compression ratio is for this engine?

If you do find it is the lower compression motor and you don't want to change the pistons, you only have to have the head shaved down a little to get it back up.
If you're rebuilding an engine anyway, you should get the head skimmed anyway, so skimming it down a few extra thou won't ad much to the cost.

I am not sure Benny, on the compression ratio.
But yeah i agree that shaving the head may bring i back to spec.
I like your way of thinking will have to speak to the man working on the head about that.

the piston rings have 2 dims on the section and you would need to check the spec as to what these are. The pistons will also have a certain ring groove dimension for compression and oil ring. The piston shape also presumably will be different for a lower comp version. You either need access to the specs or go and see one of the forum guys. Measuring bores might be an idea also to assess condition but again you might need a pro to do this for you. Its easy to spend money- if the engine is good you may well get away with little or no work to be done. If you go skimming heads you also then have to disassemble valves and get into consideration of valve seats etc etc. If the engine was good ( the one you purchased) it might have been an idea to just change it over?

I will take some pics of the pistons and post them, see if anyone can spot a difference?
Perhaps I should have just stuck this engine in, and rebuilt my old one.
But i had no idea on the condition of it, and my current engine is still going.

Seemed alot easier to just rebuild this LONG NOSE, and then take out the short nose.

but maybe not the end of the world..
I can buy new pistons from the US while our dollar is good, only other prob i believe would be the cam shafts, which i suppose i can just use the ones in my Short nose?

Make of this whatever you can. These are all the ring sets listed in a 91-92 Mazda parts book. AT always refers to auto transmission, MT to manual.

NA6 Ring sets 1.jpg

NA6 Ring sets 2.jpg

Thanks MANGA,

Kind of makes my theory a little more solid.

B6Y3 = Auto
B6Y4 = Manual

So i assume the 3 stands for mm.

[manga_blue]

Yh, manual and auto pistons have different listings too.

Do you have the parts catalogues yet? Makes re-builds a lot easier.
http://www.miata.net/garage/90-99Parts/91-92/

[Dweezle]

Yh, manual and auto pistons have different listings too.

Do you have the parts catalogues yet? Makes re-builds a lot easier.
http://www.miata.net/garage/90-99Parts/91-92/

OHHH CHAMPION!!!!!!