27 replies to this topic

[minispud]

Ok lol

[R1minimagic]

Lol, i guess all the bits i mentioned don't exist then!

[minispud]

Lol, i guess all the bits i mentioned don't exist then!

Yes, but the skirt is the whole of the side of the piston and the crown the top

[R1minimagic]

Who taught you that?!

[minispud]

If you got a short block engine and need to check to see if it needs a rebore, you don't take the pistons out, you find out the max piston-to-bore clearance from the manufacturer, then slip a feeler gauge in between the piston and the bore, so it stops as it touches the top ring on the trust side of the piston and read it's size. This will tell you how the bores are doing, baring any bad groves that you can see, if the reading is within said clearances then it's ok.

[Sprocket]

Who taught you that?!

I think minispud is making it up as he goes along

If you got a short block engine and need to check to see if it needs a rebore, you don't take the pistons out, you find out the max piston-to-bore clearance from the manufacturer, then slip a feeler gauge in between the piston and the bore, so it stops as it touches the top ring on the trust side of the piston and read it's size. This will tell you how the bores are doing, baring any bad groves that you can see, if the reading is within said clearances then it's ok.

But that is just it, you dont!!

How many times do people (read that as I) have to point out your mistake

And the only reason I am persisting wih this, is that you will tell all and sundry, how to measure piston to bore clearance incorrectly leaving a trail of mayhem behind you!!!!!!!!!!!!!!!.....................

The skirt starts after the bottom ring, the ring pack is part of the crown. I mean, the Queen doesnt just wear a thin plate of gold on Her head, does She?

Angain, in the nicest possible way, you are wrong!

[liirge]

Sprocket is correct, ive always know that bit between crown and skirt as jsut rings or ring land (probably not the right terminology), but the skirt starts after the rings..
There you go:

[Sprocket]

The bit between the crown and the skirt is technicaly called the ring belt, however, two piece pistons as used on marine and comercial diesel engines, the crown includes the ring belt and the skirt is the lower part of the piston that wears.

At no time should any part of the ring belt or crown touch the bore, it is unlubricated, and this is why pistons are considerably smaller diameter at that point. Piston to bore clearance is on the skirt, and the largest diameter would be measured at the base of the skirt at 90 degrees to the wrist pin. This is because the dimention of the piston anywhere else is smaller to accomodate different expansion rates due to the different mass of the piston construction and regional temperature variations. Basicaly, once the piston is up to running temperature it becomes more round and the skirt at the bottom 90 degrees across the pin is the part that expands the least.

[R1minimagic]

It would be analagous to a woman wearing a skirt that comes all the way up to the bottom of her boobs

[Guess-Works.com]

Ah the Simon Cowell effect

[Ethel]

The terminology won't wear out your engine but it would be useful if we all meant the same thing.

The gap at the top of the piston is sufficiently large for us to take in to account when calculating compression ratios and is even exploited in some cases to increase ring to bore contact by directing cylinder pressure to the back of the top ring. It's probably not the best place to look for signs of wear. Since the function of the piston we're interested in is to keep the cylinder pressure in the cylinder and out of the crankcase then that is what we'd be best measuring with a compression or leak down test.

A skirt is surely something that hangs down (i.e. is only attached at its top edge)?

[R1minimagic]

It is nearly always exploited in that way to push the top ring against the bore to provide the sealing mechanism otherwise the compression would be too low and blowby too high.

[Sherlock]

If you have the sealed breather system which is connected to the carburettor, the oily inlet ports suggest crank case pressure pushing oil through the breather, which is normally caused by compression ring problems.