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Quick question for the knowledgeable;

Do NB MX5 valve clearances increase or decrease from cold values as motor warms up to operating temperature?

I have aftermarket cams fitted now with smaller base circle, and cam manufacturer quoted hot specs so std mazda cold specs are no longer valid.
Just trying to work out what to set them to cold, as setting them hot is painful (literally) and inconsistent. (By the time I have car apart enough to check, it has cooled a bit so cant be sure the hot values are still valid..)

hot values would be more than when cold... gaps/tolerances generally expand as things get hot.

I'd aim for the hot value as a maximum and accept anything slightly under when cold.

Thanks gslender ,

So given hot specs are 0.008 in and 0.010 ex, what would you recommend to use as cold values?
I have no idea how much things expand, so not sure how much to subtract from hot values to get cold values. Is it 2thou or 3, or 4?

There isn't going to be a definitive answer without a lot of shop measurement and testing. As I mentioned before, I'd aim for a ceiling of the hot settings when cold (ie anything less would be changed out) and then accept anything over as ok. You'll struggle to get them 100% on those values anyway (0.2 of a mm is a small clearance to get bang on just swapping shims without shaving valve stems) - generally a greater clearance is just noisy and runs the valves cooler. If you're running race cams then it's going to get tapperty anyway, so a bit more won't be critical (the lost lift isn't a big deal if you're DIY'ing the engine build)

I've never come across any camshafts that have require hot measurements. All the ones I've played with have all require 0.2mm, and some 0.25mm cold.

project.r.racing wrote:I've never come across any camshafts that have require hot measurements. All the ones I've played with have all require 0.2mm, and some 0.25mm cold.

But what if... :

I found on my engine that clearances grew by 0.002" on intakes (.008 cold ~ 0.010 hot) and 0.003" on exhaust (0.012 cold ~ 0.015 hot)

Given your hot specs of 0.008 & 0.010 hot that would give 0.006 intake and 0.007 exhaust for cold values.