jaka

Hi!
The TT.40 PRO engine has a thick 0.40mm head shim as standard.
Do you keep that head shim when using 15% nitro or do you replace it with thinner one??

In Sweden we use 80/20 fuel in Q-500 pylon so do you have any suggestions to what shim to use then.

Giant

FOR FAI FUEL TAKE OUT THE SHIM

jaka

Hi!
Okay! So no shim is needed at all when running 0% nitro..?

fizzwater2

My Feedback: (61)

depends on the tolerances in the engines. I've seen TTPros that would almost have no head clearance at all without the shim.

Dave Shadel (www.pspec.com) used to sell an assortment of shims for the thunder tiger, you might try his website, see if he still has them. He had a range of thicknesses from .001 to .005 I believe. You could use them to build up what you need.

I've not run one on 80/20 fuel, so couldn't provide much recommendation for you.

djlyon

With 15% nitro we generally end up with .004 to .008 depending on the engine. As fizz says be careful of tolerance if you take the shim out altogether.

Denis

fizzwater2

My Feedback: (61)

You really run yours that low? I think I've been running around 13

fizzwater2

My Feedback: (61)

of course our air is a lot more humid here in Kansas than you probably are in CA - it's harder to squeeze humid air...

daven

My Feedback: (1)

I use NO head shim on the OS .46AX here with 15%.

djlyon

Yup a lot less humidity and a lot less water vapor
I've been through about 6 or 7 TTs. I break them in on the shim they come with, about .014 and then start dropping about 2 or 3 thou at a time until I find peak rpm. I use the thickest shim that gets me that rpm and seems to unload well in the air. The thinnest I have settled at is .005 and the thickest is .008. I did have one that peaked at .002 but then on the first hot day it told me "give more head room or I'm going to grenade on you"[:@] .

Denis

jaka

Hi!
Please guy's! Metiric please...much simpler that way!
I make my own aluminum shims so ...I try and run it with one 0,1mm shim first and we'll see.

djlyon

That 0,1 is a .004 Jan. But first you have to decide to use a , or . . I would start with a 0,2 and come down on it about 0,05 at a time. Using 0,1 as a starting point could cause confusion because of tolerance like fizz said.

Denis

fizzwater2

My Feedback: (61)

Denis - are you measuring actual head clearance at TDC, or only shim thickness?

That might account for some of the difference.

djlyon

Only shim thickness and based on the difference in shim thickness I see my guess is there is a lot of difference in head clearance from engine to engine, but I never measured it.

Denis

fizzwater2

My Feedback: (61)

I've found that there are a few thousandths of difference from engine to engine, even to the point of nearly having contact between the piston and head with no shim. That would explain the differences in your numbers and mine.

I don't have my notes here at work, (shh...) so I don't know for sure how thick the shims are in the three or four TTpros that I have.

jaka

Hi!
I will make a few 0.1mm shims and try what works best.

djlyon

If you measured head clearance I would be interested to know what you found ran best out of curiosity. I never measured it because there wasn't anything you could do about it except change shims anyway; rules, so I just shimmed for rpm. Sense the head on the TT40 has a tapered squish band about .004 I always kind of thought the inner edge of the squish band should wind up about .009 or so from the piston for best rpm in our rpm range but I never checked it. I always wondered why the Nelson had a squish band with no taper. It looks like a receipt for detonation. But what do I know I'm just a dumb aero guy .

Denis

fizzwater2

My Feedback: (61)

Using a flat depth mic across the head, I measured to the seating surface of the head -
Same depth mic (actually a dial depth indicator) I measured TDC - more than once. This can be tricky if the piston/sleeve fit is squeaky tight. Dub Jett recommends measuring BDC and subtracting the stroke if you know it, so that's another way.

I think my best TTpros were set up between .013 and .015 on the piston/head clearance for 15% nitro. Not too hard to needle, seem to run just fine.

vicman

My Feedback: (10)

I've noticed a .004 variance in the engines I have had over the last few years. Must be a local thing since I have had good success running .007 clearance. I check them with an indicator and Jo blocks.

GSJames

I measured the head clearance of 2 TT 40 Pros yesterday with my brand-spankin' new digital depth gauge http://www.garrettwade.com/digital-s...ge/p/96T01.12/ using 2 different techniques. In all 4 cases I got the same results. WITHOUT head shims, there was essentially ZERO clearance. If there is any clearance at all, it is on the order of 0.0005-0.001". And that's with a dead-cold engine. I have no idea what happens when the metal expands. I was planning to take out the stock shim (0.015") and run without, but based on my measurements and earlier recommendations in the forum, I put in 0.007" worth of Dave Shadel's shims. When I have time, I'll run tests with different thickness of shims but from what I measured, you had better run it with at least some shim unless you want to smack the piston into the head. [:@]