20 replies to this topic

[Bench Racer]

Within the GCR under SMCS rule 9.1.8.C.1.f.1. Cylinder Head

"The muinimum heights of the cylinder heads as measured in the factory service manual allowed are shown in th following table."

The question is, when this rule specifies factory service manual is this the same as the 1990 Mazda MX-5 Miata Workshop Manual? If the answer is yes they are one and the same there is no plane location in the Miata Workshop manual specifying to measure 5.245 inches from a specific plane to the piston side of the head/milled surface plane. Which plane is the other plane used when milling the head? Also what is the procedure for measuring?

Pretty important rule which seems to be left pretty open.

Or maybe it seems pretty open because it's the first time I'm interested in the rule.

Thanks

[Jason J Ball]

Your minimum height of the head is from valve cover surface to head gasket surface. Or the top and bottom flat surfaces of the head itself.

[Doug007]

Cylinder head inspection is on page B-48 of the 1990 factory service manual. This is the same as the workshop manual you are talking about.

The height is obviously the height from the head gasket surface to the valve cover gasket surface. I'm not sure how it could be interpreted any differently without being way off of the 5.245 measure.

Interestingly, the specified 5.245 min is 0.023in less than the minimum for replacement called for in the factory service manual (5.268). Getting to [max compression specified in the GCR] is step one in spec miata head building...

[Jim Drago]

Interestingly, the specified 5.245 min is 0.023in less than the minimum for replacement called for in the factory service manual (5.268). Getting to 5.245 is step one in spec miata head building...

Before the engine rule rewrite, their was no minimum head thickness. Builders were taking advantage of this rule by cutting the heads as much as possibly to un shroud the valves. The 5.245 is about .005 thinner than required in most cases to get to max compression without decking the block. Not sure getting the head to min thickness is the first step, we don't bring any to the minimum thickness.

I do agree it would be pretty hard to misunderstand that dimension...

[Doug007]

I should have said milling the head to get max compression is the first step...

[Bench Racer]

I do agree it would be pretty hard to misunderstand that dimension...

To my limited knowledge of the measurement process used by the SCCA to measure this 5.245 inch number I'll guarantee that there are several measurement process and each process will provide a different measured number. There is a specific reason that the process of "Geometric dimensioning and tolerancing" exists.

[Jim Drago]

Not sure what gaurentee means , but feel VERY confident that is a measurement that ANY tech official could make accurately.

[davew]

David D, give me a call. Please

Dave

[Bench Racer]

Not sure what gaurentee means ,

Basically, it means the word is miss spelled.

but feel VERY confident that is a measurement that ANY tech official could make accurately.

Really, "ANY" tech offical.

There is a specific reason that the process of "Geometric dimensioning and tolerancing" exists.

This ^ process has been around for ever & the sole purpose is so that everyone involved in manufacturing/measuring will come to the same conclusion when manufacturing or measuring an item.

[Caveman-kwebb99]

This ^ process has been around for ever & the sole purpose is so that everyone involved in manufacturing/measuring will come to the same conclusion when manufacturing or measuring an item.

Can you please say that in English? I dont understand a thing your saying...

[Bench Racer]

Can you please say that in English? I dont understand a thing your saying...

That ^ is exactly why the GD&T process was developed & introduced.

The engineering folks on this site will understand.

[Caveman-kwebb99]

That ^ is exactly why the GD&T process was developed & introduced.

The engineering folks on this site will understand.

Can I understand what your saying if I stay at the Holiday Inn Express?

[Jim Drago]

Really, "ANY" tech offical.


This ^ process has been around for ever & the sole purpose is so that everyone involved in manufacturing/measuring will come to the same conclusion when manufacturing or measuring an item.

What is a tech offical.


All ball busting aside...
If you, me, a tech official or anyone else can't accurately measure the thickness of a Miata head without reading a procedure, they shouldn't be measuring anything unless it needs to be rounded to the nearest foot, maybe yard. but as always, if you think the procedure needs to be further clarified.. You know the address, CRBSCCA.com

[Caveman-kwebb99]

I would certainly hope that any normal Joe, could measure the stupid thing, if not may as well throw the rule book in the trash. We need weight numbers on the sides of our cars....

[Bench Racer]

I would certainly hope that any normal Joe, could measure the stupid thing, if not may as well throw the rule book in the trash.

Some normal Joe's either don't or can't reafd the SMCS, hence we have all this Oscar protest crap. & you believe the normal Joe can use a measuring devise to .0001 of an inch repeateably. A tenth is a common number measured in a manufacturing shop. Please take a micrometer that measures in tenths, measure a round piece ten times with out looking at the micrometer readout, then read the measurement, mark the number on a piece of paper, repeat 9 more times & see how many times your numbers matched.

[Caveman-kwebb99]

Some normal Joe's either don't or can't reafd the SMCS, hence we have all this Oscar protest crap. & you believe the normal Joe can use a measuring devise to .0001 of an inch repeateably. A tenth is a common number measured in a manufacturing shop. Please take a micrometer that measures in tenths, measure a round piece ten times with out looking at the micrometer readout, then read the measurement, mark the number on a piece of paper, repeat 9 more times & see how many times your numbers matched.

All micrometers are not created equal, and anything I would have here would not pass any such test. I suppose it doesnt matter anyway, the only time there are major tear downs is at the major events... How many have ever been DQ'd for having a head that was shaved to much? I havent been racing SM as long as you but I have never seen or heard of it yet. Tear downs at NASA nats, SCCA runnoffs, ARRC and never heard of a DQ on head thickness. I have seen a DQ for no Heater Core though! If they caught that I am sure they would catch the over shaved head? So it is really worth discussing. Seems to me this would be caught by a whistler wouldnt it? Well maybe not if your bottom end was to worn out to build the compression.

[Caveman-kwebb99]

[Glenn]

. Seems to me this would be caught by a whistler wouldnt it? Well maybe not if your bottom end was to worn out to build the compression.

Or the "right person" was running the whistler

[Caveman-kwebb99]

Or the "right person" was running the whistler

Conspiracy Theory!!!!!! That would include all cheats if the right Tech person was looking, he could just close his eyes and give you a pass. Glenn can you measure a cylinder head and know if it is shaved to much and know for certain that the measurment you took was totally accurate?

[Jim Drago]

A head cut below minimum thickness is not something that can be caught by a whistler, a head below minimum can still be under the max allowable compression.