I am just about to start to shim the squish on my 4 valve 984. How small can I go on the squish?
It looks like the standard setting that people are using is 1mm.
What is the smallest I can safely go?
Before I started to modify my engine I had 1.03mm. With this I had just a very small buildup of carbon in the pistons so it was not far from perfect.
I would think that 1mm is good but it is not easy to get spot on 1mm. What about is I should end up on 0,9mm? Is it to small. Is it better to go for 1,1 if I dont manage to get the perfect shim?
Just for info I have 68mm stroke, carillos, pistons are 18 grans lighter than stock 916, the crank is dynamicly balanced and it is new bearings everwere.
Jocke........
__________________
MHR 900 1984
851 Strada 1991 (now growing to 996)
S4 984 2002
I am just about to start to shim the squish on my 4 valve 984. How small can I go on the squish?
It looks like the standard setting that people are using is 1mm.
What is the smallest I can safely go?
Before I started to modify my engine I had 1.03mm. With this I had just a very small buildup of carbon in the pistons so it was not far from perfect.
I would think that 1mm is good but it is not easy to get spot on 1mm. What about is I should end up on 0,9mm? Is it to small. Is it better to go for 1,1 if I dont manage to get the perfect shim?
Just for info I have 68mm stroke, carillos, pistons are 18 grans lighter than stock 916, the crank is dynamicly balanced and it is new bearings everwere.
Jocke........
I am at .86 (from 1.0ish stock). I have heard, from a non-famous engine guy Ducati pulled "in-house" to build MotoGP engines, that he has gone as low as .80mm (his eyes were gleaming when he told me that, btw). I might have liked mine a little tighter, but that is where it ended up with a .3mm DP gasket, and to mill .02 or .04mm from the cylinder base...well, I didn't think it worth the bother. I decided that would be money better spent milling the heads a wee bit....
You gotta be careful how you measure/average your readings. The piston will rock in the bore enough (if you use clay or solder) to force you to either take the closest (smallest) single number as your base, or decide that an average is your "squish." I took a conservative average, front to back, using a piece of solder just outboard of each valve....front to back, they differed by .002-3".
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i aim to run 0.90 - 1.0mm on customer engines, altho i've have gone down to 0.85 from memory when that's what limited options gave me. i'd go 0.80mm minimum i guess, 0.85 - 0.90 on my own if i had the time to really get it right.
bruce told me once (in his usual bruce way) that you can go down to 0.7mm if you warm them up before getting into it. jd progressively raised his 750 from 0.6mm or so until it stopped rattling, which from memory was about 0.80mm or so. better to ask him.
i'd definitely go for under 1.0mm tho. i get real funny about anything over 1.1mm, not really sure why. it's not that much.
you can get shims laser or water cut too - it's not that pricey.
I have the Ducati Desmoquattro Performance Handbook here, by Ian Falloon.
He says on page 41
Quote:
checked statically, this is generally around 1 mm to allow for con-rod stretch and crankcase flex. Factory figures for racing stipulate between 0.95 and 1.05 mm. If the squish is too tight and the piston touched the head, the result can be catastrophic, so this is a very critical setting.
I did a lot of experimenting when I was racing a 748. The engine morphed between 748/800/853cc over the span of a couple years. Being a 748 and a racebike... naturally the heads were off a lot let's just say. Anyway that did give me a lot of chances to try different things and see the actual results. All my figures are in inches because I'm too lazy to do the math to convert. *From memory!* .032" = uncomfortably heavy piston/head touching all around the piston. .034" = medium amount of touching. .036" = pretty much a 'nil' amount of piston/head clearance. Meaning, the squish areas were clean of carbon. When setting up customer engines I really don't like to go below .038" just for safety. (my bank account's safety, heh)
A lot of it depends on the engine component spec, and a lot depends how you measure the squish clearance too. Ex: Clay will read one thing, solder another. What type of solder are you using? Solid, flux cored? Are you "pre-flattening" it before checking, etc. My theory, I think you need to leave a little "fudge factor" with solder because it deflects the rod a small amount. I prefer to take direct readings of deck height and gasket thickness to calculate
squish clearance, assemble at a reasonably safe number, check for touching next time the engine is apart and adjust as needed. Obviously this is not a reasonable solution for street engines!
Keep in mind- These are just the lunatic ravings of an Ohio farmboy.
0.8 is as low as I would go for a race engine runing steel rods and getting regular rebuilds, and running race gas with carefully set up ignition timing.
1.00mm is perfect for a fast race engine with infrequent rebuilds (ie: once a season, say 3000 km, ) or a fast and reliable road engine running titanium rods or steel rods.
Measure it like this, using the average to identify the overall squish value. Place your 1.5mm thick solder in 4 strips, covering the front and sides of the piston crown, lightly grease the head surface to stop the solder sticking, use the old head gaskets, refit the head, torque it down to 40 Nm and rotate the engine to TDC, feel the solder crush and resist, and continue in a smooth motion until about 80° after TDC, then rotate the engine backwards over TDC and down 80°, then pull the head and measure the squish through the full length of the solder.
This can be important depending on what pistons you are using, and on how the head and its squish zones have been prepared.
Its a good idea to set cam timing and check valve pocket clearances at the same time, saves the wear and tear on the studs...
a 748 will kiss the head at 0.6mm squish at high rpm as experimented.
i havent been afraid of using 0.8 measured with 1mm thick solder although i make sure the customers of race engines get there engines right up to temputure before kaning it.
He also is the only person to mention valve to piston clearance with the proper timing that is going to be used. I use both solder and turbine blade wax that Guy Martin turned me on to. The wax is a go no go situation and comes in accurate thickness. The solder has the tendency square up the piston in the bore when compressing it, if used in the four corners of the piston, hard to do with the 748 though due to combustion chamber shape. The wax confirms things in other areas of the piston crown. Like the top of the crown to head. With a sparkplug in place you can check clearance there as well. California Cycle Works carries the wax.
.8 mm would be the limit on squish and I'd hold the valves to 1mm. This would be with quality or new main bearings as well.
Quote:
Originally Posted by Le Kiwi
Hi Jocke,
0.8 is as low as I would go for a race engine runing steel rods and getting regular rebuilds, and running race gas with carefully set up ignition timing.
1.00mm is perfect for a fast race engine with infrequent rebuilds (ie: once a season, say 3000 km, ) or a fast and reliable road engine running titanium rods or steel rods.
Measure it like this, using the average to identify the overall squish value. Place your 1.5mm thick solder in 4 strips, covering the front and sides of the piston crown, lightly grease the head surface to stop the solder sticking, use the old head gaskets, refit the head, torque it down to 40 Nm and rotate the engine to TDC, feel the solder crush and resist, and continue in a smooth motion until about 80° after TDC, then rotate the engine backwards over TDC and down 80°, then pull the head and measure the squish through the full length of the solder.
This can be important depending on what pistons you are using, and on how the head and its squish zones have been prepared.
Its a good idea to set cam timing and check valve pocket clearances at the same time, saves the wear and tear on the studs...
That squishi thing
.
Anyway that did give me a lot of chances to try different things and see the actual results. All my figures are in inches because I'm too lazy to do the math to convert. *From memory!* .032" = uncomfortably heavy piston/head touching all around the piston. .034" = medium amount of touching. .036" = pretty much a 'nil' amount of piston/head clearance. Meaning, the squish areas were clean of carbon. When setting up customer engines I really don't like to go below .038" just for safety. (my bank account's safety, heh) 
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