Is this just the way they are measured or is there a real difference? Do I need to buy a test lifter first when it comes to a race engine? Is the .904 to allow for slight oversizing with a hone?

I think most of the race solid roller "904" lifters are designed and built for Chevy motors
Call the company you like the most and ask them

If want to get the clearance exact, buy the lifters you intend to use. Then fit each one individually. They range from .902-.904. They will even vary in a set. Personally mine are pretty sloppy, up to .0025". It has caused zero issues in my motor.
Doug

Yeah, 0.001" difference.

The official size for Mopar engines is 0.904"

Unless you have a rigid hone in that size range you can't do much good with a brake cylinder hone. That's my story and I'm sticking to it.

R.

There are legit differences, depending upon the brand/manufacturer of the lifters.

Not a bad idea, especially if you're trying to fit the lifters to lifter bushings, etc. The recommended spec, IIRC, for roller lifters in iron or aluminum blocks is .002", which is a crap shoot if one brand measures .9031" and another measures .9038"... which is exactly what I found myself.

FWIW, I went w/ the .9038" brand, but my lifter bores are still too loose due to previous wear w/ the bronze bushings.

I just measured a set of solids. Largest was .9036", smallest .9034" not too shabby.

This is why I am asking. I have seen both 903 and 904 listed for Mopar. Cab says 904 is Chevy. Dogdays says 904 is Mopar. Maybe it doesn't matter. Just whichever one fits best?
Is there a minimum and maximum clearance you guys like using before a lifter either galls the bore or bleeds off too much oil pressure? Or do they chatter around if too loose? My Indy block does not have any lifter oiling. Do specs change for that?

The hole size is nominal, so .904 is Chrysler. Not Chevy.

The lifter size SHOULD be made with the clearance built into it. Remember A-B=C?

A is hole size. .904
B is lifter size. .903
C is clearance. .001

I know some of the lifters are coming .904 and I ASSume it's so you can fit them but it's about impossible to hone a hole straight and round that's that small and short. A couple of tenths maybe.

I do the Hughes "thunk" test to see if they are too tight. If too loose there will be low oil psi problems (what I am dealing with right now) & I have purchased some .001 OS lifters & they all mike out to .905". 360 non roller.

The only thing you need to know is the size of your lifter bore and the size of your lifters... mix and match as needed among one set or bush the bore if you want an exact fit.

Agreed. There is no .904 " Chevy" lifter. That is a Mopar size, that Chevy guys, and Ford as well, have adapted to take better advantage of the ramps/lobes, and robustness as well.There is no standard among lifter manufacters, you kind of need to know who does what with their particular brand. I have a drawer full of almost every lifter I can imagine, just for sizing purposes, and even then, they may change and be more than .0005 off when you get a new set. I often have semi custom lifters made, to better size for an existing block that has been bushed ( or not) and can dial in the sizing better if they arent going to re-bush. I can also re-hone a bushed block, get a semi custom, and save a lot of extra cost for the customer.Bottom line, my opinion,get the lifters you plan in using, then size accordingly, UNLESS< you already have a known
bore issue ( size or roundness).

My Chrysler Service Manuals lists the lifters at Ø0.9040 to Ø0.9045".
Lifter bores to be Ø0.9050" to Ø0.9058".

You will find that can be from dead nuts on, to over .0025 off, there just isnt a " norm".Many lifters measure .903, or .903 and change. Some measure .9045. Some, within the same set, arent within hardly .0005 of each other. Its a crapshoot.