FaanP

Thanks for all the replies .... I now feel a bit more comfortable buying the motor. Can you guys give me some direction on where to get one - I'm struggling to get some answers out of the online hobby store where I wanted to buy it - thus I think I must look for a different supplier.

keavze

anyone knows what's the head shims to have for the DSIII zx21 using 25%?

OliverT

Hi,

there are so much factors affecting head clearance, but 0.5 should be a good starting point.

M7H

Don't mix both...
to get 0.5mm head clearance, you in general have to put in 0.3mm of head shims.... But I have seen head buttons of 0.2mm and of 0.3mm (distance between piston and head button when put into the liner with no head shims)
The best is to measure it, don't do this with a brand new engine, but you can do it after the first gallon. (to much pinch when they are new)

Remove the glow plug, turn the flywheel and put the piston at TDC, only then unscrew the cooling head and head button (otherwise you will push the liner up)
Now take your caliper and measure the distance between top of piston and top of liner. also measure the distance the head button goes into the liner (remove all shims!) the difference is the head clearance the engine has without shims (0.2/0.3mm)...

Don't forget to turn the flywheel, to get the piston out of TDC!

Roelof

Just use a piece of solder.

Bend it in an L-shape, put it through the plughole, turn the flywheel arround so the piece of solder will be squeezed between head and piston.

Messure the squeezed solder, that is the head clearance.

keavze

so am i right to say that for .21 engines, i would need 0.5mm of actual head clearance to use 25% at sea-level with airtemp 80-90F and humidity 70-80%?

M7H

That should be right yes, maybe 0.1mm more and run a richer setting at that temp/humidity.....

Roelof

Different brands have a different headclearance so maybe it is better to say Picco .21 engines have a clearance of 0.5mm ?

afm

From my experience:

Minimum effective clearance (piston to head) on any .12 engine is 0.4mm
Maximum effective clearance is 0.8mm

Minimum effective clearance for any.21-.32 engine 0.50-0.55mm
Maximum effective clearance 1.1-1.3mm

Rule I've always gone by, on the safe side is:

0.60 on a .21 running 25 - 30% nitro and a #7 plug

0.50 on a .12 running 16 - 20% and a #6 plug


Relative humidity on ambient. The more humidity (more water on the air), more chamber is needed. I'll add one 0.10mm shim for each 20% it raises over 70%. I use a small weather station for measuring temp, barometric pressure and relative humidity on the track. It is a great tool to know the quality of the air your engine is breathing.

Height over sea level. As you go higher, there is less oxygen on the air, so you have to reduce space on combustion chamber (more compression). The more you approach sea level, you need to make more space for the additional Oxygen present in the air.

AFM

xspower

iTrader: (4)

Please read AFM's post carefully. The clearance is to be measured as stated piston to head. Not just shim thickness. Big difference when it comes down to thousandths of inch or hundredth's of a MM. The quickest way to check the clearance from the head to the piston is to crush some solder and measure the part that was crushed. The most accurate is to measure all and then do the math I have found no difference if the solder technique is done correctly.

keavze

hi guys,
anyone knows the difference in engine characteristics/performance between the zx21 DS3 and DS4? or rather between the .21 picco evo3 and evo4?

the 4 has better topend but the 3 has better lowend?

p.s. forget the title...typo error

xspower

iTrader: (4)

The crankcases of the Mega's are completely different in volume to the Picco case. You should have some more top end with the Mega's as the case volume is slightly higher. The 4's for both have a under head that is fashioned after the Nova with the o ring support.

Scott Fisher

iTrader: (28)

Still doing engines Jerry?

xspower

iTrader: (4)

Yes Scott.