Hi All,

I have done a search but can't find any definitive answers, any help would be greatly appreciated.

I've bought a spare engine and am rebuilding the head to put on to my forged block. The head currently on the engine has leaking valve stem seals and rattles.

The new head is getting OEM valve stem seals and gasket, it's been skimmed (decked) by 6thou /0.15mm which I am told is the minimum the machinest could do. Unfortunately the machinest numbered the lifters and shims but not the valves, therefore upon reassembly and checking the clearances pretty much everything is out of spec.

I have lapped in all the valves and taken measurements, however the Miata engine guide I have says to measure with cylinder 1 & 4 at TDC - therefore the lifter to cam clearance at 90° and 270°, but elsewhere on the internet it states to position the cam 180° away from the lifter (heel of cam to lifter clearance). Assume 0° is valve depressed and angles are clockwise from there.

I've ended up taking all 3 measurements and they've come out somewhat different. To my mind the only clearance I care about is at 90° & 270°, where the cam releases and hits the lifter, right? If this is the case should I disregard the 180° measurement? As long as the cam isn't touching at 180° (which it isn't) this shouldn't matter, right?

I am keen to get a nice, quiet running engine that doesn't smoke so need to get this right!

Questions:-
Should I ignore the 180° clearance measurement, even if it's different?
Will an OEM headgasket be okay with a 6thou skim?
Should I set the clearances in the middle of tolerance or bias towards tight/loose?

N.B. - '99 car, Turbo'd running 264hp, forged rods.

I've attached the measurements for reference

Thanks in advance

Wilson

 

I always measure 180 from the lobe. Gasket should be fine.

 

I do mine with the cam lobe @ 180° so I'm sure I'm at the base circle of the cam. That's just the way I learned.

Whatever is the least amount to take off to make the head flat is the best.

From your spreadsheet, Intake 2 & Exhaust 2 need to be reshimmed and potentially Intake 5 & Exhaust 1.

FYI, following is a list of the available shims now from Mazda.

Z502-12-545 3.052MM
Z502-12-552 3.100MM
Z502-12-558 3.148MM
Z502-12-565 3.196MM
Z502-12-573 3.252MM
Z502-12-579 3.300MM
Z502-12-586 3.348MM
Z502-12-593 3.396MM
Z5Y1-12-541 2.750MM
Z5Y2-12-541 2.800MM
Z5Y3-12-541 2.850MM
Z5Y4-12-541 2.900MM
Z5Y5-12-541 2.950MM
Z5Y6-12-541 3.000MM

 

Thanks Bahurd

I was taught the 180° method as well. However, with Mazda stating the 90° & 270° degree method, as well as the fact that the clearances actually increase at 90° & 270° I'm thinking the base circle isn't important (that bits not going to contact and therefore not make any noise). If the tolerances at 90° & 270° were tighter than 180° I would agree with your method.

Thus, either my cams are knackered, with wear at 90° & 270° (I don't believe this is the case and I have seen the same situation on another forum), or the 90° & 270° bits are the important bit to measure(?)

In which case, should I be setting them to nominal or tight/loose?

Thanks for the part numbers 👍

​​​​​Wilson

 

With all due respect, you're talking about trying to set clearance using a set of feeler gauges that is basically a go/nogo situation (.001") and you want to further tighten the tolerance by setting "tight/loose" where there's only a .002" tolerance anyway. And all this where you have a .002" (0.050mm) variation in what you measure.

 

To add to the list

 

Good point - although it is possible depending what the starting clearance is. I'll settle for anywhere in the range then (as that's what the range is for). I am keen that this head doesn't rattle for many miles though, the current racket my engine makes is unbearable, only further embarrassed by plumes of blue smoke on startup.

For clarity - Mazda Dealer couldn't confirm - Black valves are inlet - Green are exhaust - correct?

Anyone got any other views on measurement strategy - 90 v 180 v 270?

Thanks

Wilson

 

I posted the Mazda part numbers in another forum here. Here:https://www.miataturbo.net/engine-pe...2/#post1414251 post #37.

No idea what color is what but you can compare part numbers.

 

I saw that thread - thanks for the link.
I went chasing the part number rabbit and came up empty besides one seller on eBay that was selling a set of 8 Kia branded valves for $10.

I bit the bullet and ended up spending ~$300 on a full engine gasket kit. Which they managed to put all the stem seals in one bag with no part numbers - great work Mazda.

 

Black is intake.

 

I might've missed the info, but can you play the mix & match game with what you've got?

Start with valve 1, and start going through your collection of existing shims until you're in tolerance. Repeat with valve 2, etc.
Eventually you'll run out of shims, if you keep track where in the range some of the previous ones landed, you could swap further. Eventually in theory you'll be left with just a few shims that need ordering, rather than every single one.

 

That's the normal way of doing it...

 

That is what I have done,

By mixing and matching I have managed (on excel at least) to get everything to nominal (or tight-end of spec on the intake side) and I only need to buy 2 shims.
The issue, in my head, is if I use the 180° measurements as opposed to the 90° or 270° it affects the re-shimming strategy. Thus far I am using the 270° clearances as that is directly before the valve is activated/depressed.

 

The goal of the cam manufacturer was to have a constant radius everywhere except at the lobes. The fact that you get different readings is noise, or imperfections; and no conclusions can be drawn.

The man with two watches never knows what time it is.

 

If they were both smart watches they would read the same.

 

In which case I shall take the 270° readings as the critical measurement. They are ultimately the clearances that matter before the lobe hits the shim. My logic says the tighter I make this gap (within spec) the quieter the engine should be.

The readings at 90° are important to the valve closing noise but are overall fairly similar to the 270° measurements.

Wilson

 

Its a cold clearance though. Its a calculated number based on how much the clearances will tighten up. So it depends on what position the number was calculated based off of.

 

Interesting thought, but the manual says to check it at these angles (whether that's for ease / efficiency or not / assuming it's perfectly circular - I don't know). I would have thought that 99% of all the thermal expansion is dealt with in the valve stem / shim bucket arrangement. I wouldn't have thought the cam would change too much - maybe I'm wrong on that one.

On the whole, my clearances with the lobe at 180° facing away from the shim are greater than the clearances at 90° & 270°, minus a few outliers, therefore the thermal expansion is still accounted for by getting the 90/270 clearances within spec.

I'll try to get some shims and hoping to get the head fitted on Monday so will report back.

Wilson

 

I can't read gud so I just followed pictures.



That being said the FSM says to put the cylinder at TDC. Does not mention the angle of the cam at all.

 

Haha - Fair play.

That image does show the lobe at 180°, but that's only applicable to the valves on cylinders 2&3 when at TDC on 1 & 4.

The valves on 1&4 will be horizontal when at TDC.