piston replacement

[John]

Renewing a piston over the weekend, are there any tricks apart from straight forward assembly. For instance what is this breaking glaze on the bore or any other not obvious stuff I should do?

[Jolly_MG_Maestro]

Just renewing 1 piston can be iffy, for a start you don't know how much wear over standard the bore has unless you have the bore measured, in which case if the bore was worn to oversize you would have to buy oversize rings to match which come in a set for 4 pistons, placing rings on a piston is an art in itself without snapping them and the pistons themselves need to be spotless and would ideally need to be cleaned by submersing in an alkali acid to burn off all carbon which would then allow the rings to seal properly on the piston, and you need to deglaze the bore with a "honing" tool, basically usually a 3 leg device with an abrasive block on each leg which can be attached to a drill and as the tool goes around you need to go up and down the bore with it to produce angled scores in the bore, then to insert the piston you need a ring compressor around the piston to compress the rings against the piston as it wont go in the bore without this, make sure that the bore is oiled for the piston to side in and make sure the compressor is lightly oiled inside for lubrication and not too tight around the piston, place over the bore and tap the piston into the bore with a hammer handle, and you may have to make sure the piston is the correct way around, also if you're going to disturb the bottom end it may also be worth changing the big end bearings, personally if your going to do it for one, do it for them all, or better still get the bores checked properly and bored out to the next oversize with correct rings, best way to go IMO ;)

Get it wrong and you could end up with a smokey engine or slight miss with lack of compression etc.

A job like this always leads onto other jobs, so a full rebuild is sometimes the best option instead of cutting corners

[John]

There's only one thou wear on the bore.
I'm fitting new rings as well, it was a broken ring that damaged the piston and Ive got a ring compressor.
The honing tool is the only thing I'm short of, see if I can borrow one. Thanks for the advice.