Calan
DSM Wiseman
- 7,251
- 362
- Jan 16, 2007
-
OKC,
Oklahoma
Brian (19Eclipse90) and I spent a while tinkering with my t-belt last night after replacing the head, and we noticed something a bit odd was happening.
Before dropping the head on, I was rotating the crank and checking things over, and noticed that when the pistons were at absolute top dead center, the notch on the crank plate was about 5* (1/2 tooth) past the pointer that is cast into the front case. I never noticed it when the motor was on the stand, but I never checked it that precisely either. With the pointer lined up exactly, the piston has just barely reached the top of the stroke, if that.
So anyway, I didn't think too much about it, and we installed and tensioned the belt. I used a straight edge to make sure the cams were dead on, and then locked them down with one of Jay White's cam gear locking tools. I worked from the cam gears going clockwise, and around the tensioner pulley last as usual. Once we FINALLY got the tensioner pulley in the right place (it was being a PITA for some reason), the tensioner is where it should be and passes all the usual tests (drill bit, grenade pin slips in and out,. etc).
Here's the kicker... no matter how many times we attempted to re-time it and play with belt tension, the crank sprocket always ends up at that 5* ATDC point when everything lines up and is "comfortable". Here is roughly what it looks like:
When I try to go an extra tooth to correct for it, I end up with the crank BTDC, and the pointer looks like this:
Trying to gain a tooth on the intake or exhaust cam gear just throws the cams out of time with each other, and doesn't help things at the crank.
So, I have two questions...
1. Considering only the short block, how come when the #1 piston is at true TDC (slightly after max travel but before any downward travel), the crank plate doesn't line up? Is it just a fact of life for the stroker geometry, and I've just never noticed it?
2. Once the motor is timed, which way will the crank sprocket try to rotate as you increase or decrease distance from the cam centerlines to the crank? At first my thought was that the crank was rotating slightly CW because I have decked the block and shaved the head, but... I am running a thicker HG now which should more than offset that; so I would think the crank would want to sit a little BTDC if anything.
Thoughts?
Before dropping the head on, I was rotating the crank and checking things over, and noticed that when the pistons were at absolute top dead center, the notch on the crank plate was about 5* (1/2 tooth) past the pointer that is cast into the front case. I never noticed it when the motor was on the stand, but I never checked it that precisely either. With the pointer lined up exactly, the piston has just barely reached the top of the stroke, if that.
So anyway, I didn't think too much about it, and we installed and tensioned the belt. I used a straight edge to make sure the cams were dead on, and then locked them down with one of Jay White's cam gear locking tools. I worked from the cam gears going clockwise, and around the tensioner pulley last as usual. Once we FINALLY got the tensioner pulley in the right place (it was being a PITA for some reason), the tensioner is where it should be and passes all the usual tests (drill bit, grenade pin slips in and out,. etc).
Here's the kicker... no matter how many times we attempted to re-time it and play with belt tension, the crank sprocket always ends up at that 5* ATDC point when everything lines up and is "comfortable". Here is roughly what it looks like:
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When I try to go an extra tooth to correct for it, I end up with the crank BTDC, and the pointer looks like this:
You must be logged in to view this image or video.
Trying to gain a tooth on the intake or exhaust cam gear just throws the cams out of time with each other, and doesn't help things at the crank.
So, I have two questions...
1. Considering only the short block, how come when the #1 piston is at true TDC (slightly after max travel but before any downward travel), the crank plate doesn't line up? Is it just a fact of life for the stroker geometry, and I've just never noticed it?
2. Once the motor is timed, which way will the crank sprocket try to rotate as you increase or decrease distance from the cam centerlines to the crank? At first my thought was that the crank was rotating slightly CW because I have decked the block and shaved the head, but... I am running a thicker HG now which should more than offset that; so I would think the crank would want to sit a little BTDC if anything.
Thoughts?
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