Forum whore
Forum whore
There is one detail missing in those recommendations: the number of seizures.Originally Posted by Javier Ruda
Aprilia must have made a fortune selling cylinders and pistons in 1995....
Highway Biker
With a 69.5mm bore and forged wiseizcos I use .0055”. This is because when the engine dyno operator gets out of a pull they sometimes stick if any tighter. Before we dyno tested everything, there was no issues at the track with tighter clearance.
Scooter boy
Indeed, these Aprilia numbers are way off compared to the others.
L-Plate Rider
I have the info that with that tight piston clearance (2/100mm) they had to heat up first the coolant liquid and the entire engine before they could even start the engine.
Hope my info is correct.
Forum whore
Condyn the only time I have seen anything like 0.0055" used ( 0.1375mm ) is in hotrod jetskis , due to the very cold waterjacket temps used.
The fix is to use restrictor pills in the water dump tubes from the cylinder , or a thermostat to keep the water temp at a reasonably sane level.
If any race engine locks up when backing off a load , then that instantly red flags the tube/needle is way lean on less than WOT.
The Wossners in 72mm size for a CPI 485 Banshee I have built probably 20 of these, and they run forever at the factory recommended 0.06mm ( 0.0024" ).
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Hardcore Biker
Aprilia made a fortune on selling pistons every year....
Piston clearance should be 0.04mm they said.
Out of curiosity I tested bigger clearances:
0.05mm gave MORE power.
And 0.06mm gave the same power as 0.04!
There were 'heavy' and 'light' pistons.
With the heavy ones you could do all practice sessions AND the race without power loss
The light ones had to be changed every day, they deformed and caused power loss...
I never understood the 'advantage' of the lighter pistons.
Except that more money was made on them.....
On the dyno I always used the heavy ones, without any problem!!!
Up to 30 power curves with the same piston, without any power loss!
About 80% profit was made on the pistons....
Hardcore Biker
The 'pockets' are mostly negative for power.
Cast pistons give slightly more power in my experience!
Scooter boy
Thank you Jan! Then cast and 0.05 mm is the good try, and no pockets as andreas and wobbly said.
I have one more doubt, maybe silly. The cylinder is not new, so a bit ovalised, 0.006 mm. Does this make a difference?
Forum whore
I diamond hone when I see 0.005mm , its always on the thrust face and each side on the bridges.
But the KZ engines run 0.07mm clearance to start with to reduce run in time ( thus extending crank life ) and there is no difference in power I can see between 0.07 and 0.09.
You can do it a couple of times and fit a new piston, then go up to the next size from 53,93 to 53 ,94.
If you dont hone it , then you see brown blowby just below the ring very quickly even with a new piston fitted.
Edit = 0.005 correction.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Scooter boy
Thank you wobbly! By the way, very impressed from that measurement.
Forum whore
Sorry one to many 0s - 0.005
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Highway Biker
Re- wobbly on the clearance.
I will take a closer look at the needle/tube, however this issue has only happened on the dyno and never at the track thus far with slightly tighter bores. Also I have never noticed anything that stands out on my data logger on shutdown.
We race in temperatures sometimes cresting -30c so I am sure that is why the manufacturer specs such a loose clearance. .004-.006” or .1-.15 mm is very common to see recommended in the snow application. Some oem cast pistons are specified a tighter clearance than that, however not very common. By no means am I saying the common practices that we have been using for decades are correct, they may very well be folklore. The risk, or imaginary risk of seizing at the track is just too great to tighten the clearance, especially after the dyno experiences.
Forum whore
Yea I get it now - in the snow you have the same issue as the jetski"s except its the ambient air freezing everything.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Forum whore
The 360 is up and running again with a new autolube system. Seems also the disc had shifted again, this is why it started to behave odd on the dyno, also becoming difficult to start. Disc drive system was only ever clamped in place but now is pinned. Sounds better, starts easier and apears to hold tune now. Pictured a squirrel fan for extra cooling, zippy tied on.
Maybe now some results to 'take home'.
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