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I dusted off my AutoCAD skills and modeled my '85 and '86 cylinder's port timings using the 79mm stroke and the 139mm rod length (early models). I measured the top of the exh and transfer ports from the deck. This calculation is based on a 0.0 deck height. This is for anyone's information who is interested.

Seems my previous confusion was over how much the piston actually moves at a particular crank rotation. 90* crank rotation does not equal 1/2 of the stroke. Creating the visual model solves all the confusion. Here's what AutoCAD is telling me from the model and my measurements of two stock ported jugs. BTW, both of these jugs have 90.5mm bores, so the exhaust in particular will be more mild than stock due to the angle of the port roof.

'85
exhaust opens at 47mm (+-0.1mm) = 174* duration
transfers open at 67 mm (+-0.1mm) = 106* duration
blowdown = 34*

'86
exhaust is the same 174* duration
transfers open at 66 mm (+-0.1mm) = 110* duration
blowdown = 32*

190* exhaust timing would require cutting the port roof up to 41.73mm, which I'm quickly losing the appetite to do, preferring something more moderate.

Cheers!

i dont recall a stock jug being 0 on the deck with a .020 oem gasket. seems like it was in the hole atleast .010 or even more. been a while since i checked so i could be wrong.

The 0 deck height is theorhetical -- I'll cut the cylinder on mine so that it zeroes out. I guess the port timing will be slightly more aggressive on a stocker if the piston doesn't come all the way up.

plynn41 wrote:Seems my previous confusion was over how much the piston actually moves at a particular crank rotation....

I printed out a degree wheel on paper and taped it to the flywheel and had some dead acurate numbers, no reason to spend that much time modeling it out.

on my 86 the deck is at 0 with a cometic, not on any of my other engines without work

Thanks Roosty--I'll hope for the same.

To Danny, I spent all of 20 minutes doing it, and my cases aren't assembled. I think it would take me 20 minutes to find TDC if the engine was assembled. The only error will be in my ability to measure and accurately cut the ports--the calculations are very accurate.

dannygraves wrote:I printed out a degree wheel on paper and taped it to the flywheel and had some dead acurate numbers, no reason to spend that much time modeling it out.

yep. made one out of totinos pizza box lid. still have it and use it to this day.

plynn41 wrote:Thanks Roosty--I'll hope for the same.

To Danny, I spent all of 20 minutes doing it, and my cases aren't assembled. I think it would take me 20 minutes to find TDC if the engine was assembled. The only error will be in my ability to measure and accurately cut the ports--the calculations are very accurate.

without a dial gauge through the spark plug hole your chances of ever finding "true" TDC are slim to none because the dwell time will fuck you up. you can eyeball it and get close, ive done it before.

suppose you could also use a dial gauge mounted on a stand if you have a way to secure it 100%, but if it moves even the smallest amount your measurements will be off.

100hp honda wrote:without a dial gauge through the spark plug hole your chances of ever finding "true" TDC are slim to none

Your so fuckin' stupid....

Take ANY OBJECT.... like a 14mm socket.

Stick it on top of the piston and turn the motor clockwise until the socket is flush with the deck. Mark the degree wheel.

Turn it past TDC, then turn it counter clockwise until the socket is again flush with the deck and again mark the degree wheel.

The point between the two marks is true TDC.

No fancy tools, no math. Just intelengence, something your obviously missing.

Go sit in your snow plow, and wait for winter you dumb mother fucker.

AlisoBob wrote:

100hp honda wrote:without a dial gauge through the spark plug hole your chances of ever finding "true" TDC are slim to none

Go sit in your snow plow, and wait for winter you dumb mother fucker.

AlisoBob wrote:

100hp honda wrote:without a dial gauge through the spark plug hole your chances of ever finding "true" TDC are slim to none

Your so fuckin' stupid....

Take ANY OBJECT.... like a 14mm socket.

Stick it on top of the piston and turn the motor clockwise until the socket is flush with the deck. Mark the degree wheel.

Turn it past TDC, then turn it counter clockwise until the socket is again flush with the deck and again mark the degree wheel.

The point between the two marks is true TDC.

No fancy tools, no math. Just intelengence, something your obviously missing.

Go sit in your snow plow, and wait for winter you dumb mother fucker.

holly smokes batman!

bob, rather than go to the trouble of using a 14mm socket and making sure its certered perfectly on top of the piston, wouldnt it be easier to put a straight edge across the top of the jug and roll the piston over until it contacts the straight edge ?? bet you never thought of that eh einstien. still the most accurate way is dial gauge.

100hp honda wrote:bob, rather than go to the trouble of using a 14mm socket and making sure its certered perfectly on top of the piston, wouldnt it be easier to put a straight edge across the top of the jug and roll the piston over until it contacts the straight edge ?? bet you never thought of that eh einstien. still the most accurate way is dial gauge.

What are you doing to account for the dome in the piston?

Dome/dish/hole makes no difference as long as whatever is used sits in the same spot throughout the test.

Unless I'm missing something, laying a straight edge across the top of the jug and rolling the piston around will just move the staight edge out of the way when at TDC.

Ahhh I see what your saying now, I thought you were referring to bobs post about a rod of somekind in the sparkplug hole.

what your doing is rolling the piston up until it barely touches the straight edge, put mark on the degree wheel. roll it past tdc and the roll it backwards until barely touching straightedge, put mark on degree wheel. this is essentially the same as bobs method, only easier

100hp honda wrote:bet you never thought of that eh einstien.

Didnt think you owned a straight edge. They cost $5.

Also, The farther you get away from TDC, the more accurate you are.

Maybe you could pop one of the windsheild wipers off your parked plow, and use that as your straight edge.

Taxpayer dollars at work.

i use my 6" caliper for the straight edge with it next to 2 studs opposite from each other and take a flange nut and finger tighten the straight edge.

Then like bob said spin the degree wheel until it hits and make note of the number then spin the motor the opposite way and split the differance between the two.

example: if it reads 26* before TDC and 12* after TDC move the pointer (or move the wheel) so it is at 19*

make sure you double check yourself and roll the motor both ways so it is showing 19* before and after TDC.

then remove the straight edge and now the piston can be moved to TDC and it will be exact.

I prefer this method over any other way of finding TDC

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