piston movement??

[blingsta]

Can anyone tell me how much movement you can expect from a piston? i mean while the heads off and you try to move it from side to side? reason i ask is that i changed the piston rings about 1000 miles ago and while ive had the head off ive checked the pistons for movement and i could physically move them from side to side!!! but i wasnt getting any smoke on idle or on drive before head came off. is this normal?

[maxfield]

There shouldn't be no movement.

[blingsta]

Hmmm.. ive got a bit of movement on all of them!

[Sooty]

There shouldn't be no movement.

Sorry m8 but your wrong .
If there were no clearance at all between the pistons & bores, then how would they be able to move up & down?

The actual piston to cylinder clearance will depend upon wear & usage, etc, but should be:
New 0.02 - 0.05mm (.0008 - .0020 in)
Wear limit 0.15mm (.0060 in)

Measurements are usually taken 10mm from the top & bottom of the cyl bores & around the circumference of the piston in line with the gudgeon pin, so trying to measure the gap around the top land of the piston is a waste of time.

As long as you can see the crosshatching of the hone marks on the cylinder walls & your not burning oil then you should be alright.

HTH m8

Iain S

[miamistu]

There shouldn't be a vast amount of movement, but there will be a clearance from the piston to bore wall which gets bigger the colder the engine.

That's why we have piston rings, to take up the gap!

[maxfield]

When i did work experience the mechanic there said there should be no movement So he must be rong.

[blingsta]

Sooty, whats the crosshatchings on the hone marks look like??

[Brianmoooore]

Sooty, whats the crosshatchings on the hone marks look like??

If you can see them, you'll know why they're called cross hatching.
There's normally a perceptible sideways movement in a piston on a cold engine.

[Martinaston]

Just check to see if the bores have gone oval if they have then its best just to get a new block.
If they are still ok then check they are not beyond the maximum wear limit.

[Sooty]

When i did work experience the mechanic there said there should be no movement So he must be rong.

Yep !!!!!!!!!

Just check to see if the bores have gone oval

He's only going to be able to check that with a bore gauge m8. Not the sort of instrument found in your average cantilever toolbox

If they are still ok then check they are not beyond the maximum wear limit.

Read my paragraph about measurements again & ask yourself how he's going to do that with the pistons in place.

[Demlotcrew]

Remember that you should measure the piston size about a third of an inch from the skirt.

Most pistons rock at TDC.

Remember that the top (crown) of the piston expands more because the metal there is thicker.


But i agree with Sooty, the specs he gave are spot on.


Andrew

[kam-325i]

Some movement in the pstions at TDC on a cold engine is to be expected, dont forget, metal expands when hot....

[bigkev]

the alloy pistons have a greater rate of expansion to cast iron or steel liners/block so they can not be the same size hence the slight movement this is why most engine wear takes place when cold as the pistons rock slightly until up to temp

[Brianmoooore]

the alloy pistons have a greater rate of expansion to cast iron or steel liners/block so they can not be the same size hence the slight movement this is why most engine wear takes place when cold as the pistons rock slightly until up to temp

Engine wear takes place when cold because that low octane petrol stuff most of you run your cars on condenses out on the cylinder walls and washes the oil film off. That's why petrol cars need a richer mixture when cold and LPG cars don't.

[chu346]

Would 97octane be better?

[Brianmoooore]

Not in the slightest. Unmodified E30s cannot gain the slightest advantage from using any fuel with more than the minimum octane rating required to prevent detonation.

[reggid]

the alloy pistons have a greater rate of expansion to cast iron or steel liners/block so they can not be the same size hence the slight movement this is why most engine wear takes place when cold as the pistons rock slightly until up to temp

Engine wear takes place when cold because that low octane petrol stuff most of you run your cars on condenses out on the cylinder walls and washes the oil film off. That's why petrol cars need a richer mixture when cold and LPG cars don't.

the reason for a rich cold mixture is that the fuel has 100's of different chemical components which all ignite at different temperatures. At cold temps only a certain components of the fuel burn, call it component X. So the mixture needs to be rich so that there is enough of the component X to run properly.

[reggid]

Not in the slightest. Unmodified E30s cannot gain the slightest advantage from using any fuel with more than the minimum octane rating required to prevent detonation.

adding to that higher octane fuels often have a lower lower heating value i.e. less energy per unit of mass so you may lose a hp or so.

[Martinaston]

sooty, it was a bit pointless posting the measurements then

[blingsta]

cheers guys..

[Sooty]

sooty, it was a bit pointless posting the measurements then

The point my friend, was to show that there is a clearly defined measurement of clearance allowing movement between the piston & bore as opposed to the hearsay of inexperienced people.
Something you obviously failed to appreciate.

[TURBODOCTOR]

im new to e30's but im not so new to engine building. can anyone tell me how the hell a piston can move freely up and down a bore or liner if its made size and size? thaught not! if someting is made size n size then it is a press fit is it not?

[kam-325i]

if someting is made size n size then it is a press fit is it not?

No...Thats an "Interference fit" or " They fucked it up again" !!!!

[Sooty]

No...Thats an "Interference fit" or " They fucked it up again" !!!!

[Globulator]

i changed the piston rings about 1000 miles ago and while ive had the head off ive checked the pistons

I have been studying recently some interesting threads about running in piston rings, the key seems to be about getting some torque through the engine in the first 20 miles so you grind them in properly .

http://mototuneusa.com/thanx.htm

There is also some interesting stuff about flow turbulence for tuning that I may well apply to my spare 325 head I aquired!