Speeding Along Now
Posted 28 March 2015 - 11:37 AM
my 998 block is bored out to +60 and I have some +60 nural flat top pistons.
yesterday I put the top of the piston in the block to check the gap and I found there was quite a bit of movement , I measured the block and it came out at 2.583" over the four and the pistons measure at 2.572, I was just wondering if this is an acceptable size difference?
I also put the ring in by itself and it fitted well with a ring gap of .011, is this also within spec?
many thanks
Up Into Fourth
Posted 28 March 2015 - 12:01 PM
Piston to bore clearance has to be wider at top of piston to allow for expansion, they get hot. For this reason they are machined with taper, and also some ovality.
The reference point to measure piston to bore clearance is at the base of the piston at 90 degrees to the gudgeon pin.
Ring gap sounds about right, would not want any less particularly for the top ring.
Gearbox Guru
Posted 28 March 2015 - 12:20 PM
OVAL ??? it'd be fecking difficult to make a bore or piston oval, intentionally.
the rule of thumb for ring gaps is 0.004 to 0.005 thou gap per inch of bore, so you're ok with that, and if they are the correct rings for the piston then you should be good to go.
Ideally you should get the block machined to the piston as not all pistons are the same and may have small variations in bore diameter requirements, and can therefore lead to tight or sloppy pistons.
Up Into Fourth
Posted 28 March 2015 - 03:28 PM
Yes, just about all pistons are slightly oval.
Not a lot, just so that when at running temperature the piston to bore clearances are optimised.
The bores are all perfectly round, or should be... The only company I can think of who tried out oval bores for a while was Honda on their 500cc V4 racer back in the 80's. They went back to round bores, so I guess this was not a success.
Speeding Along Now
Posted 28 March 2015 - 07:14 PM
OVAL ??? it'd be fecking difficult to make a bore or piston oval, intentionally.
the rule of thumb for ring gaps is 0.004 to 0.005 thou gap per inch of bore, so you're ok with that, and if they are the correct rings for the piston then you should be good to go.
Ideally you should get the block machined to the piston as not all pistons are the same and may have small variations in bore diameter requirements, and can therefore lead to tight or sloppy pistons.
do you think the clearance from bore and piston is ok then? they are the rings that came with the piston
Uncle Cooperman, Voted Mr TMF 2011
Posted 28 March 2015 - 07:28 PM
Sounds fine, but check the ring gaps by removing the rings (carefully!) and sliding them down the bores in turn. Ring gap should be between 0.003" and 0.005" per 1" of bore diameter, so 0.009" to 0.015" is fine. If the gap is slightly larger, maybe 0.017"-ish it will still be OK.
Moved Into The Garage
Posted 28 March 2015 - 08:12 PM
(hope you can read that OK)
Speeding Along Now
Posted 28 March 2015 - 08:52 PM
Sounds fine, but check the ring gaps by removing the rings (carefully!) and sliding them down the bores in turn. Ring gap should be between 0.003" and 0.005" per 1" of bore diameter, so 0.009" to 0.015" is fine. If the gap is slightly larger, maybe 0.017"-ish it will still be OK.
that is exactly how I did it! I was surprised how well it slid up and down in the bore and maintained the same gap, and yes the gap was .011
cheers for the info ;)
Edited by kyle9832, 28 March 2015 - 08:55 PM.
Speeding Along Now
Posted 28 March 2015 - 08:54 PM
(hope you can read that OK)
A very helpful picture there, cheers for that :)
Moved Into The Garage
Posted 28 March 2015 - 09:14 PM
Yes, just about all pistons are slightly oval.
Not a lot, just so that when at running temperature the piston to bore clearances are optimised.
The bores are all perfectly round, or should be... The only company I can think of who tried out oval bores for a while was Honda on their 500cc V4 racer back in the 80's. They went back to round bores, so I guess this was not a success.
Carbon is on the money here, no question in the world! Where there's more metal, the pistons will expand the most from ambient to running temps.
So, the Crown of the piston is a fair bit smaller as not only is there more metal there, but it's also subject to higher heat than the skirt.
Likewise, the Gudgon Pin Bosses have more metal here so they also expand more than the skirt either side of them, though these days, there is a tend to go to wasted skirts, so it is difficult to pick up that this is the case.
The process for manufacturing the pistons to this shape is refered to as 'Cam Ground' though once they were actually ground to shape and size, these days it's done by CNC.
Sometimes too, odd shapes are manufactured in to the piston shape for thrust compensation (thrust ve non-thrust) and this is why some pistons specifically state which way around (ie FRONT or an Arrow) they have to be fitted in to the engine, sometimes these also have off set Pin Bores too. Another reason is for coff coff silent running. An example of this is the early 850 pistons which had a spilt skirt, though I recon it was a governor of sorts.
Edited by Moke Spider, 28 March 2015 - 09:14 PM.
Up Into Fourth
Posted 29 March 2015 - 11:20 AM
Ok abit worried here, no one has mentioned your sizes do not add up. a +0.060 bore on a 998 engine would be 2.603" How are you measuring the sizes? Also you need to check the piston to bore clearance at the bottom of the skirt for a fast road build I would be looking at 0.0025" more for more rpm. Bore must be checked with micrometer equipment such as telescopics, bore mics, and my factorite the dial bore gauge. Anything else is not good enough.
As already suggested, yes pistons are manufactured oval and tapered.
AC
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