Did you read the rest of it? It says what cylinder should be at TDC for each measurement.

 

Per that diagram, cylinders 2 & 3 are measured at cam positions that are 90 cam degrees out from the relative lobe positions 1 & 4 are measured at.

So it seems that what Mazda is saying is that none of this matters.

 

Yup - If cylinders 1 & 4 are at TDC on their respective compression strokes (360° out from each other), the valves of cylinders 2 & 3 will always be horizontal to the shim.

If the cam was perfectly round for the base circle then I would agree it doesn't matter, the fact mine are not introduces error into the equation which ultimately will put my clearance out of spec depending which measurement angle I go with.

 

Sounds like you need a new cam then...

 

Meh, the engine ran fine in the car it came out of and looks well looked after, so I'll rebuild this head and see how it goes. I'll also measure my old head to see why that is making so much noise.

Figures like mine aren't out of the ordinary, when people check at all 3 positions. The lack of information is because most people, like yourself, use the age old method of checking at 180° rather than the strange Mazda method. This issue arises (3 separate threads) when people go back and check the book to find they measured at the 'wrong' orientations and do it again, only to find different numbers.

I believe / hope if you were to check your clearances at 90/180/270° you would also end up with varying clearances.

I think I'll stick with the 270° clearances and build from them. Now I'm hoping my old head has a 3.2 & a 3.25mm shim in it as Mazda UK can't get hold of any until next week.

 

I've been adjusting shim style bucket set ups for 3 decades. Safest bet is to check adjustment at TDC compression stroke on each individual cylinder. This is proper and will work great on everything from 4 cyl to V12...
It always puts the valves for that cylinder on the base circle.
Every base circle I've ever checked has been concentric and the old time method for 1/2 intake and 1/3 exhaust then rotate 360 and get the others works fine WITH STOCK CAMS for 4 cylinders.
I looked at your spread sheet and you actually show greater clearances at 90/270 which is exactly opposite of what I expected you would see there...
I expected a tiny amount tighter as you might be getting into the lead in/exit ramp (you will be in this area with a cam of longer than stock duration).
You had extra clearance!!!
I still suggest 180 degrees or TDC on each individual cylinder but because of your post but next time I have a Miata cam out I'm going to put it on V-blocks and check the base circle with a dial indicator.
Base circles almost never wear. Every one I've ever checked has been perfect but your post shows a deviation that should not be there.
On most bucket style set ups changing the valve adjustment .001 changes the cam opening and closing points approximately 2.5 degrees.
Mazda's method checks at different points on the base circle (thus meaning Mazda also thinks the base circle doesn't vary).
You want to be safe?? Check and adjust each valve at the exact same point. TDC is easiest.
Don't have nice mark on balancer to show TDC on 2/3 use a dress tape measure to make a mark 180 degrees from the original mark (after you have verified the factory indicator is correct)
Allow no variation.
Your current (FSM) method does not do this.
Hope this helps.
Rick

 

The base circle on my cams don't vary, I've checked them. That part of the cam doesn't touch anything, so I don't see how it can wear. I swapped out an exhaust cam that had been damaged once, and didn't have to change any of the shims, all the clearances were still good.

AIUI, the minimum gap is there to allow for thermal expansion, to ensure that the tolerances don't go to zero and leave a valve hanging open when it's not supposed to. If so, then to be "safe" all three base circle measurements should be at least the minimum.

OTOH, the only reason I can think of for the base circle being inconsistent is a manufacturing defect, such as if the cam profile was ground incorrectly and it isn't centered on the longitudinal axis of the camshaft. If clearance is smaller at the 180 degree point than at 90 and 270 then perhaps the profile is shifted in that direction. That would mean that the cam will produce less lift than it's supposed to.

You might want to measure diameter of the cam at the max lift point and compare it to another cam that doesn't show the differences in clearances. If it's the same diameter then you know it's been shifted.

Easiest way to find TDC on cylinders other than #1 is to stick a long screwdriver down the spark plug hole. Slowly move the crankshaft and you can watch it go up, stop, and start to go down. That'll get you within a few degrees easily.

--Ian

 

Thanks for the replies folks.
If I'm not mistaken though, TDC on a compression stroke puts the intake valve at 90° and the exhaust valve at 270° doesn't it?

I've re-shimmed and managed to get the vast majority in spec at all angles, annoyingly I have a couple of valves way below threshold so looks like I have to pull the cam again :-/

Wilson

 

These 90 and 270 numbers mean nothing and are kind of confusing. What are you getting at? Since the Mazda procedure puts the lifter at radically different parts of the base circle for cylinders 2-3 vs 1-4, these angles aren't significant, especially since they aren't all that close to 90 or 270 degrees in reality.

At compression TDC, if the intake lobe center were 90 degrees from the lifter axis, that would mean that peak lift is at BDC of the intake stroke. In reality it's more like 55 degrees.

 

What I was getting at is Mazda set it to check at these angles as it shouldn't matter (assume ease of checking rather than continually rotating the cams). In my case however that is a fallacy given my clearance do change depening on angle. Whether that means the cam is knackered or not, I do not know. What I do know is that I am not the first person to run into this issue and both my current (now old) head and new head both displayed this behaviour to some degree.

Having said that. In the end I managed to get pretty much everything within spec, albeit at opposing ends of the spec between 90 and 270 degrees. It's all back on and is significantly quieter now. Although I do still have a knocking noise which I refuse to believe is a big end so will be investigating noisy rx8 injectors and/or knackered HT leads.

Thanks for the input

Wilson