Need an idea for a good improvised straight edge.

Pulled the head off one of my vehicles today,need to straight edge it. I have a good straight edge,somewhere. I'm going to attempt to find it tomorrow,but if anyone has any ideas,in case I can't find it, I'd like to hear them. Now for those of you unfamiliar,I'm looking for .004 or less warp on the head,so it has to be a very straight,straight edge,not just a yard stick or something. Kind of thinking of using dental floss,stretched tight over identical height spacers and using a 3rd spacer like a go no/go gauge if it goes under the floss with a gap or won't go under its no go. Any ideas? Hoping I find my straight edge.

Glass/mirror

Quality level (Stabila) and a feeler gauge maybe

Robert Bare said:

Glass/mirror

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Do you think the edge of a glass pane is perfectly straight?

check it with a string
Or find a metal rod, or wood dowel. Test by rolling it.

.004 is pretty tight.
Decent bubble level.
Engineer or architect scale.
Drywall square
Big speed square or framing square
String line is always straight. I’d be thinking fishing line. Tough part is knowing if you’re hitting the line or if there’s a gap of a few thou.
Something rigid is better to bump the feeler gauge.

And idk what you’d find that is the same dimension to the thou basically. If you have 2 points gap sets could use those to shim the line up.
Straight cue stick, broom handle or rigid conduit or pipe should be very straight as well. Just harder the check. And not necessarily straight to .01 much less .001.
Some small aluminum tubing or angle (nice sharp square edges)
Bet the top edge of a long sawzall blade (a new one) or hand saw is very straight.
Long drill bit.
Upside down hand saw might be the best straight edge if you don’t have a framing square.

Lol u do that for a living, huh?

Problem solved,was walking to the barn determined to find my straight edge and walked right past a tempered glass pane for an entertainment center. Walked over to the cylinder head placed it on edge on the head saw a gap,rotated the glass 90* still had a gap all in all any which way I stood the glass on the head I had a gap dead center of the head.Measured it with feeler gauges .005+. Just for fun because I didn't know if the glass was a for sure straight edge,I grabbed a piece of dental floss pushed it to the head on either end with my index fingers stretched it tight and looked,you could look right under the dental floss. That's all the confirmation I needed it's loaded it up to get it resurfaced. 3 minutes instead of hours searching the barn. FWIW it's amazing how obvious the warp was using the dental floss.

Rub it on the curb a little bit - it'll be alright!

Rusty Nail said:

Rub it on the curb a little bit - it'll be alright!

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"slightly advanced"