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I'm in the process of replacing my cam, inner cam bearing, lifters, cuffs, and I'll be replacing my oil pump with the updated Harley oil pump on my 2018 Heritage Classic. I'll be using SE adjustable pushrods.
So like I always do, I put my thumb over the spark plug hole (rear cyl in this case) until I feel compression, then I insert a plastic straw to find TDC. I move to the other side of the bike and I can spin the push rods on the rear cyl with my thumb and forefinger, good to go. I'll borrow a set of bolt cutters tomorrow.
Just for the heck of it, I aligned the timing marks across from each other which to me would mean TDC, but it's not. I assume that when the dots line up that's when you have ignition (spark)?
Here is a pic of my timing marks after I've verified TDC on the compression stroke of my rear cyl.
AFIK, it's not necessary for the pistons to be at TDC to align the cams properly. On my Sportster, the most important positioning of the pinion shaft mark has to only point at the #2 cam center point, which puts the pistons about half way up in the cylinders. I would not worry about it, but check your FSM to be sure. Your engine most likely has a different procedure.
And when your spark happens is related to your cam timing, but not precisely as spark timing needs change with RPM and engine load. Every system is separate, but synchronized. A symphony of mechanics and electrics.
If you're installing new cams, make sure you know exactly how to align them properly, that's a mission critical job to do correctly.
John
Last edited by John Harper; Feb 26, 2026 at 08:25 PM.
Sorry John, not sure how that relates to my question. Piston has to be at top dead center and push rods unloaded so I can cut them with the bolt cutters. I just wondered why the timing marks weren't aligned when it was at top to center.
180 degrees off? Compression or exhaust stroke? Maybe using the front cylinder instead of the rear.
Just my morning swag....
Good morning CB. If it was on the exhaust stroke, the exhaust valve would be open and I wouldn't be able to spin the push rod.
I just wondered why the timing marks didn't lineup at TDC. I'm OK to cut the push rods, I can spin them, piston is definitely at TDC, valves are closed.
Adjust rear cylinder when dots are closest to each other, (dot on cam at 6 o'clock, dot on crank at 12 o'clock).
That's what I always thought, but when I do that the rear cylinder is not at TDC. It's only at TDC when dots are as shown in the picture.
I know you have to line up the dots to time the cam to the crank, but obviously that's not for TDC, if it was, we wouldn't have to bother with the straw in the spark plug hole. We would just go by the dots. I'm just having a brain fart. I always thought that timing mark alignment meant 6-12 = rear TDC and 12-6 = front TDC
That's what I always thought, but when I do that the rear cylinder is not at TDC. It's only at TDC when dots are as shown in the picture.
I know you have to line up the dots to time the cam to the crank, but obviously that's not for TDC, if it was, we wouldn't have to bother with the straw in the spark plug hole. We would just go by the dots. I'm just having a brain fart. I always thought that timing mark alignment meant 6-12 = rear TDC and 12-6 = front TDC
Have you pulled the gears off the crank and cam yet? The only way you can have what you describe is if you have something like this
And tdc on the front is not at 12-6. Tdc on the front is when the dot on the crank is like this.
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TDC Question
