8mm vs 5/16" valve stem

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Pretty darned close.
5/16" + 7.937mm
 
8mm = 0.3149"
5/16= 0.3125"
Yep,interchangeable. Comp Cams will tell you the same thing
 
Maistrelli said:
Seems to me, 5/16" stem in guide reamed for 8mm is gonna have excessive clearance. 8mm valve in 5/16 guide, too tight.
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If your 5/16 guide was a little worn/loose wouldn't a 8mm stem tighten things up?
 
Along those same lines, I wonder how many guides get hit with a 8mm. reamer and new valves installed.
 
toolmanmike said:
If your 5/16 guide was a little worn/loose wouldn't a 8mm stem tighten things up?
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New head casting. Appears to be 5/16". Have a ball gauge coming and I'll know for sure.
No way they are interchangeable if you want correct clearance. Maybe why I hear of stuck valves.
 
They are interchangable as long as the guides are properly reamed or replaced like they should be done when rebuilding a head.

Now if you're doing a regrind only no they are not interchangable.
 
It’s pretty basic really......

A lot of 5/16 valves have a stem diameter of .3110.
Which would provide .0015 clearance if used in a .3125 guide.

The typical 8mm valve stem diameter is .3130, which won’t even fit in 5/16” guide that’s sized for .0015 clearance with a .3110 valve.

How is that “interchangeable”?

It’s really more of a case of “some modification required”.
 
PRH said:
It’s pretty basic really......

A lot of 5/16 valves have a stem diameter of .3110.
Which would provide .0015 clearance if used in a .3125 guide.

The typical 8mm valve stem diameter is .3130, which won’t even fit in 5/16” guide that’s sized for .0015 clearance with a .3110 valve.

How is that “interchangeable”?

It’s really more of a case of “some modification required”.
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If you size the guides to fit the valve stem it's business as usual. Some shops will take the extra time to do this... some don't. Usually not needed in a stock rebuild. Race engine is different.
 
It’s actually almost comical that people are arguing over this.

5/16 is not the same as 8mm.

I can hear the conversation now.......
“I ran out of 5/16” guides for your head, so I used 8mm instead.
Yeh, there’s an extra .0025 clearance....... but I read on the internet that 5/16 and 8mm are interchangeable, and I didn’t bother fitting the guides to the valves because you don’t have a race motor”.
 
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PRH said:
It’s actually almost comical that people are arguing over this.

5/16 is not that same as 8mm.

I can hear the conversation now.......
“I ran out of 5/16” guides for your head, so I used 8mm instead.
Yeh, there’s an extra .0025 clearance....... but I read on the internet that 5/16 and 8mm are interchangeable, and I didn’t bother fitting the guides to the valves because you don’t have a race motor”.
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I agree. Another reason Jimmy Carter screwed the country with his shift to the metric system. We are so ignorant we can't do fractions so we decided to dumb ourselves down with the metric system.

8mm and 5/16 are NOT the same. That's why we have the decimal system. That's why zero has a value.

Just because Dave Hughes said it don't make it right.
 
PRH said:
It’s actually almost comical that people are arguing over this.

5/16 is not that same as 8mm.

I can hear the conversation now.......
“I ran out of 5/16” guides for your head, so I used 8mm instead.
Yeh, there’s an extra .0025 clearance....... but I read on the internet that 5/16 and 8mm are interchangeable, and I didn’t bother fitting the guides to the valves because you don’t have a race motor”.
Click to expand...

It is basic but it can get confusing which is why I like to have all parts in front of me to measure and do the math. For example Ferrea doesn't know what a 5/16" stem is supposed to measure --every valve I ever got from them measured .308"-.3085" and that is not even close to 5/16". So if a guy never saw that before he would assume the aftermarket calls a .308" stem a 5/16" stem valve size. J.Rob
 
I had 8mm valves put in my magnum heads and my machinist had to size all the guides. He said it didnt take much, but they did have to be sized.
 
Close but no cigar. Not without modifications.
 
"Magnum engines and A-engines do not use the same valves. Because the diameters are slightly different, the Magnum uses a slightly shorter valve by about .060 inch. It also uses a thinner valvestem. They are not interchangeable."
 
From Hughes, 8mm intake valve #1038a =.311" OD, 8mm exhaust valve #1048 =.312" O.D.
From me, new EQ head casting guide 3115" I.D.
Need a reamer or maybe two. My valves are back ordered. Have to wait and see what diameters I get.
 
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PRH said:
It’s pretty basic really......

A lot of 5/16 valves have a stem diameter of .3110.
Which would provide .0015 clearance if used in a .3125 guide.

The typical 8mm valve stem diameter is .3130, which won’t even fit in 5/16” guide that’s sized for .0015 clearance with a .3110 valve.

How is that “interchangeable”?

It’s really more of a case of “some modification required”.
Click to expand...

It's not. End of story.
 
RAMM said:
It is basic but it can get confusing which is why I like to have all parts in front of me to measure and do the math. For example Ferrea doesn't know what a 5/16" stem is supposed to measure --every valve I ever got from them measured .308"-.3085" and that is not even close to 5/16". So if a guy never saw that before he would assume the aftermarket calls a .308" stem a 5/16" stem valve size. J.Rob
Click to expand...
Thanks for this about Ferrea valves, I just ordered 4 replacment valves for a Magnum head with 5/16" stems that didn't fit. Measured valve stems installed at 0.308". Now I know where those valves came from.
 
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