piston rings

[grainger]

oh poo ...

ive just been scraping the carbon off my pistons and i realize number 2 IS a little loose in the bore. i was hoping the oiling up was just from the top ... so it looks like i DO need to take that piston out. can i just do one piston ? the others all seem solid. wriggling the big end on number 2 shows it does seem to have some play. i presume i will need to replace the big end bearings, and the little end bearing ? how do i figure out what size the piston is so i can get the right rings ?

there is no ring around the top of any of the cylinders so i presume that at least is a good sign ? also strangely number 2 had very little carbon on it compared to the others

cheers
grainger

[Cam]

You CAN do just one piston but I would do all 4 while I was at it. Also check the bores for excessive wear and hone/rebore if needed. I would also do the main/big end bearings while I was at it.

how do i figure out what size the piston is so i can get the right rings ?

You need to check the bore first to see if it's worn/scored and needs a rebore. Get a micrometer (internal and external) to check the bore and piston.

there is no ring around the top of any of the cylinders so i presume that at least is a good sign ?

If you mean no little 'lip' at the top of the bore then yes it's a good sign.

If the bores measure OK then re-hone them and get new pistons with rings.

If you are not sure then strip the block and give it to an engineering firm to check out and rebore/supply pistons and rings if needed.

[Alec]

Hello Grainger,
you will normally feel a little play of the piston in the bore, and an easy judge of wear is to feel the top of the cylinder, if worn (and they probably will all be the same) you will feel a step. It is possible to just do a re-ring and you will get many more miles out of the engine. When you have cleaned the piston top and you can't see something like +20, +30 then they are standard bore. For the price you might as well change the big end shells, they also will have a size stamped on them, little ends will most likely be OK, just feel for up and down play. if you do re ring only you will need to de glaze the bore to allow the rings to bed in. You can buy de-glazers to fit into an electrice drill, this is essential. I would not replace the piston(s) in the same bore unless there is NO discernable wear as the top ring will hit the step and break. Replacement rings have a step to give clearance to this ridge.

Alec

[grainger]

hi

im tempted just to get my cylinder head finished and put it all together, the play in the piston is VERY small. i got underneath and wiggled the big ends, there is a miniscule amount of play in at least 2 of them, and none in the actual crank, but as there is no wear in the cylinders i presume that it cant be too bad.

really i would like to change all the big end bearings, the main crank bearings, AND the piston rings all in one go which i cant afford for a while, so i think i will leave it this week and just get it through its mot

so the overbore size would be stamped on the piston crown ? well i cant see anything (except MOWOG, FRONT and the cylinder number) so i presume they are standard size which means it hasnt been rebored i take it. and as it doesnt have a lip on any of the cylinders i presume it hasnt done too many miles ... the cylinders do look very clean and shiny (is that the 'glazing' ?), if the rings were gone wouldnt there be blowback marks down the cylinder wall ?

thanks for your help
i'll let you know how i get on

grainger

[rayofleamington]

It would be worth to get the bores measured, just in case the one bore is oversize, ar all the bores have been done etc... This could only have happened from a cowboy repair but if they were cowboys, absolutely any combination of pistons / bores / bodges is possible. It is best to be very inquisitive as this piston has less coke - maybe it was messed with before.
Also the rings could be faulty, assembled wrongly or the ring gaps could have lined up, causing leakage.

A set of big end shells is probaby a good investment even if the crank is slightly worn as this will reduce noise and give a major improvement on oil pressure. I't best to inspect the crank before doing that.

If you end up needing a rebore / pistons or reground crank, a good second hand engine is a cheaper / quicker option.

[grainger]

hi

ive checked the pistons again there is definately no overbore markings on any of them. they all have the number 2 on it, which is one of the original sizes. im wondering if this piston doesnt have much carbon because its been soaked in oil a lot ??? its brown and more discoloured than the others ...

ps are the pistons made of anodised aluminium ? because the only metal polish i have says its not to be used on anodised aluminium. it says in the manual if you polish them the carbon wont stick so easily.

cheers
grainger

[rayofleamington]

they aren't anodised