NMPRA

fizzwater2 wrote:I think you'll find that the top of the piston is quite a ways further down the sleeve when it closes the exhaust port than .151, .154, etc.

The numbers that people throw out are measurements of the difference between the top of the piston at top dead center, and the top of the sleeve flange. Yes, larger numbers imply that the sleeve is higher, which raises the edge of the exhaust (and intake) ports with respect to the centerline of the crankshaft, which increases the timing duration in degrees.. so it's a way of getting relative numbers without using a degree wheel. The degree wheel would be the proper way to measure actual port timing.

and the .015 is the distance between the top of the piston at TDC, and the squish band of the head.

Thank you Gordon.

Yes, the distance between the top of the piston when the exhaust port closes and the top of the liner will not be the same as the TDC measurement I understand (and I suppose is more precisely measured with a degree wheel as you say). That's kind of why I used the term "code" meaning, when using the TDC measurement, we are really using it as a relative reference to the timing.

Tim

I guess more simply stated, when using the value for whatever TDC is (.151, .154, whatever), we don’t really care that this is a measurement of the distance between the top of the piston and the top of the liner. As Gordon suggested, it’s really just relative figure that we collectively understand is in reference to the timing.

Or (again), TDC is relative to, or a reference to the timing.

This does assume that all else (crank, rod, piston, liner, etc.) is equal (which it may not necessarily be) which is why a degree wheel is suggested. But this is not my concern here. I was just trying to understand the reference.

Tim

sounds like you got it - the first number does relate to the port timing, the 2nd number the head clearance.

Hi Tim,

Not sure what the TDC number is referring to. I usually use deck height which I interpret as distance from top of piston to top of liner with piston at top dead center (I think my motors are usually in the .187 range +/- without shims if I remember correctly) and head clearance which I interpret as the deck height less the head button depth (usually/always .185) plus any head and liner shims you might add. I think I usually start around .190/.020 and adjust from there. I have heard of others going radically different in both directions with success (probably why my stuff is never crazy fast!!)

On a motor without shims, that measures .187 deck height and with a .185 head, I add .003 under liner and .015 under head to get my .190/.020

When using a timing wheel, I measure from exhaust port closed to exhaust port closed going through bottom dead center. My motors are usually 197-199 degrees if I remember right. I have not touched a motor since the NATS, so my numbers may be way off....I have a tendency to forget things

I may be way wrong here and am happy to be corrected. Curious what the .151-.154 numbers are referring to?

Hmmm... TDC is exactly that. Top Dead Center. When the piston has reached the top of its travel. It can be measured with a degree wheel (the hard way) or measured like discussed in this thread a few pages back. This measurement is discussed only to find the clearance between the piston and head. Anything to do with exhaust timing is done with a degree wheel and has nothing to do with TDC, we'll at least not in this discussion. If you choose to measure stroke, then you can add that number to the dope book on that particular motor. EDIT. The dwell comment was not as accurate as I wanted it to be. The dwell is the amount of time the piston spends at TDC. Finding more dwell would mean finding the longest connecting rod via machine tollerences, plus the crank with the longer throw.

RR

Craig, Tim was running a Jett engine this past weekend and their deck height number is in that range.

Ahhh, got it Lonnie. That makes sense now.

I heard the race was a blast! I will make it out there one year

Ah yes, of course.

I insist that I understand the premise, but in my first post I wrote TDC in a few instances when I really meant deck height....

As Ray corrected me, the deck height is the term I should have been using, not TDC when referring to the timing. So let me correct one of my statements:

...an engine with say, a .151” DECK HEIGHT (not TDC as I originally wrote) will be “timed” lower than an engine with say, a .154” DECK HEIGHT (not TDC), right? So again, DECK HEIGHT (not TDC) in this case is really used as a reference to the timing, right?

I also understand that some insist on or prefer to use a degree wheel to measure the timing instead of going by the deck height which roughly assumes that all engine parts are the same (which could give a false assumption about the timing).

Anyway though, I was just trying to connect the fact that, when we speak of deck height we are really referring to the timing (though again, I know deck height is also a figure needed to measure head clearance).

And yea, the .151/.015 figures are for a Jett.

I suppose I've embarrassed myself here a little, but I've learned something (and maybe an anonymous reader too?).

Tim

Hey Tim,

You haven't embarrassed yourself. It is hard to always be on the same page when we aren't even sitting in the same room together.

Kurt

Hi Tim, hello everyone,

I think I'll chime in here. You cannot accurately measure the deck height unless you first accurately find TDC.

This is difficult to do because of the taper in the liner. As the piston moves up in the bore towards TDC it
tries to push the liner up also, sometimes giving erroneous readings.

Whatever method you use to check timing it is paramount that you accurately find TDC first.

Bruce

Bruce Coffey wrote:Hi Tim, hello everyone,

I think I'll chime in here. You cannot accurately measure the deck height unless you first accurately find TDC.

This is difficult to do because of the taper in the liner. As the piston moves up in the bore towards TDC it
tries to push the liner up also, sometimes giving erroneous readings.

Whatever method you use to check timing it is paramount that you accurately find TDC first.

Bruce

On the jett motor, the liner sits in a recess. It's still possible to measure the rest of the travel upwards by first measuring the gap between the case and liner with a feeler gauge then subtract that number from your deck down to the piston. This number is your unloaded TDC.

Checking for TDC
I received this motor from xxxxxxx. He asked me to go through the numbers so he had a baseline. It's a good time to revisit a simple way to find true TDC using head shims. After lubing with castor, I put the piston to TDC with the head bolted down. Then I removed the head. With the head shims provided with the motor I used them as feeler gauges. By lightly rocking the prop back and forth while trying to slip the shim between the gap of the cylinder and case. The .003 cylinder shim slipped right in, next was the .005 It fit but with a little resistance. When using a feeler gauge you don't want to force the gauge into the gap. You want it to just slide in. In the case of this motor with the piston at TDC I could still fit a .005 shim into the gap. Trying to add another .001 it didn't want to fit, so, this is the number you must subtract from your depth micrometer to find TDC. In this case the depth is .189 MINUS the .005 shim which leaves .184 TDC. Minus the head .185 which leaves a -001. There's a .003 tolerance on the big end of the rod, if you take that into account when using the shim to LIFT the cylinder you have added that tollerence.

Now that we have over twenty pages of "set-up numbers" maybe it's time to look at why some numbers work better for some than others. Operating temperature is something that should be closely payed attention to. Von Der Hey

how did i get to -001. well, when i checked the exhaust timing on this motor it was 199 degrees. this is with no shims at 199. remember .003 for every 1 degree. so this motor was .006 higher than a motor with a exhaust timing of 197 degrees.

Lee VDH, you're up. this thread needs some new input.

RR

I have some technical info to add to the thread, although it may be difficult for some to digest...a Nelson will run with the sleeve 180 degrees from where it should be, and as proven today, 30 degrees from where it should be. It runs, but not very good. Obviously my wife was watching tv when she was putting my engines back together...

Kurt