Posted From: 126.96.36.199
|Posted on Saturday, 27 September, 2014 - 08:08: |
Piston rings hold the compression and control oil. All piston rings contribute to oil control the main oil control is the bottom set just above the piston pin.
To check piston rings for wear two measurements are relevant, the width and the radial thickness. The radial thickness is measured with a ball brg and micrometer. Often measurement is not required because the radial thickness opposite the ring gap is obviously less by sight and the ring is scrap. The width is no so obvious because the width wears even.
Oil cannot get past between the ring and the bore. If the ring side clearance in the piston is slack the ring will move up and down in the piston and pump oil around the back to above the piston.
When fitting in an engine the gaps should be staggered so that a line of oil cannot be left by all the gaps being in line. Rings turn in the grooves so it doesn't need to be exact. Two stroke have pegged rings to stop turning and the ring end getting caught in a cylinder wall port.
Most rings are marked top.
Modern rings are ready gapped at manufacture. Ring gapping is really for older cars which had pep pistons and less than accurate bores. However if the rings are off the pistons by fitting a ring in the bore can be used to measure bore wear by dividing the gap by 3.142. Pi. Do not fit chrome rings to chrome bores.
Pistons generally come with rings and piston pin and circlips. The ring gap should be correct. Which saves taking rings off pistons. Eight pistons means 32 rings plus 8 oil control spacer rings. Twist or break any ring and a new set of rings will be required. When fitting new rings to old pistons check the bottom of the groove for carbon. Aluminum wheel cleaner will soften. Do not remove soft aluminum from the groove sides because ring flutter will pump oil.
Bores wear mainly at the top. If the cross hatch honing marks are still visible then the bores are good. One thou per inch of bore dia is about the limit. Which is on a 4" bore 2 thou deep and a slight ridge at the top. Using a rotary glaze buster a few up and downs will radius the ridge edge so that the New top ring won't catch should it be slightly higher due to manufacturing tolorances.
Glaze busters look like a bog brush with stone balls. Run at 300 rpm and keep going up and down to give a 45 degree pattern don't dwell in one place 1 minute only per bore using paraffin as lubrication. The buster is removing metal.
Wet liners are used because the wall thickness can be finely controlled.because they are held top and bottom they sort of float allowing close bore to piston clearance. Engines with out liners tend to have bores that curve when the engine is hot which needs larger piston to bore clearance and extra noise. Which if excessive is called piston slap.
Renault use a loose fitted liner that can easily be removed by hand. Which makes rebuilding a piece of cake. Pistons and liners for a average 4 cylinder engine are about £100(1992 prices)
The liners are set up with paper gaskets at the bottom and a bead of silicone.
RR is much more complicated and because of this it's not a diy job unless one has an oven to heat the block to 150 c and the height has to be checked for the correct nip. Which if wrong the liner will have to be removed.
Because of crude between the bottom o rings only every other liner can be removed because the block will have no support from an adjacent liner unless a liner wall support Jack is used, which is a rare piece of tooling. If the block cracks then one is in real trouble. This bit is for a machine shop with the correct equipment experience and knowledge.
However I know of two engines that have had all the liners taken out using a big propane torch a fridge, a liner jack and a dummy liner.
Chrome or nicasel bores are a pain because they must not be busted or any thing else and there are no technics and one cannot get away with anything. PORSCHE 928.!
(Message approved by david_gore)
Posted From: 188.8.131.52
|Posted on Sunday, 28 September, 2014 - 06:45: |
A bit I left out because a good film started on the telly.
Running in New bores and pistons.
Pistons don't need running in and can be max out within 1 hour of running.
Piston rings do need running in.
the rings are rubbing over 45 degree cross hatch honing marks. If the rings are heavily loaded by making the engine pull hard the extra combustion pressure will force the rings harder into the cross hatching. This can break rings if the ring digs in. Also the rings can scuff the bores and tram line them not deep just scratches which damages the rings and piston skirts.
On a zero miles engine if the oil filler cap is opened one can hear the rings shousing up and down the bores at idle. When that noise goes the engine is ready for max power. This takes about 500 miles.
The bore ring area is the only bit of a modern engine that needs running in. Obviously the rest of the engine is connected to the rings so those bits get run in by default sort of. Need to be careful with the camshaft still though for the first few minutes of start up.
All that is necessary is drive gently for 500 miles then keep to the speed limits and drive as normal.
As the miles build the bores will polish themselves seal better and the engine will give more power and feel happy.
Then as more miles build the engine has settled down and runs happy for years then sadly it starts to settle down a bit to much and the engine is worn.
The life of most modern engines is 200,000 miles if properly serviced. Some engines such as Mercedes, Rolls-Royce Nissan 2.4 diesel can go to 400,000 and a Volvo car has done 650,000 miles.
A RR V8 will happily run for many miles burning oil and rattling its tappets. It won't breakdown providing it is serviced properly and someone does not put flushing oils in the engine to unstick the tappets. Because the flushing will remove the carbon seal from the rings and the.engine will smoke bad until it's rebuilt.
(Message approved by david_gore)
Post Number: 658
|Posted on Sunday, 28 September, 2014 - 20:00: |
I don't know if RR/B rings are the same, but most aftermarket (oversize) ring sets have a top ring which has a step on the upper surface to avoid catching on the ridge at the top of the bore.
Considering the cost of a ring set for a Shadow I can only hope that I never have to fit one, but at least I've already got a ring compressor and glaze buster lurking in a drawer in my new workshop.
Posted From: 184.108.40.206
|Posted on Monday, 29 September, 2014 - 05:34: |
Ridge dodgers. These where made by cord pistons originally. Also taper faced rings for quick bedding in.
Pistons and piston rings is a whole engineering discipline.
(Message approved by david_gore)