NR 21 5 Port Head Clearance Setting

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Hydro Junkie

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Oct 2, 2006
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I just picked up my third one of these and am curious if changing clearances is advisable or even needed. Reason I asked is I printed out the exploded diagram for the T21-5M/2 and it shows two .30mm and one .10mm shim installed but with optional shims of .15, .20 and .25mm as "optional". Since I'll be running 50% nitro in sport boats, do I need to change anything? I haven't checked the actual clearances yet but can if needed for clarification

Thanks
 
so what compression ratio are you guys running with .006-.008 squish.

I have a Nova Keep 21 that stock had a DC of.023.

Ran it in according to their recommendations and now running 30%

I want to eventually run 50%

Checked some CR's last night with a 1ml graduated pipette.

at .023DC got 8.99:1 Trapped from top of exhaust

at .015 DC got 10.627:1 CR

at .013DC got 11.085:1 CR

If I run .006 with this head I'm gonna fry plugs and risk detonation arn't I?
 
Actually you will burn less plugs as the engine will not have to work as hard being optimized to produce max HP. You will want to run the engine on the rich side, as that is a large part of the cooling.

If you really want to calculate Trapped CR, I would suggest the range of 10.25:1 to 10.75:1. The 21 engines love 60%-65% nitro also.....
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