I have just installed a set of new valve lifters from Performance VH on my 90 Q45 and have discovered that they are not all at the same installed height. Upon opening the packages some of the lifters were slightly compressible while others were not, although they were all pretty close to equal in height. The factory service manual states to bleed any lifters that can be compressed 1mm or more - I bled every one that was compressible by hand, following the procedure in the FSM. I don't know if the height difference is between ones I bled or not as I didn't keep track of which ones were bled and which weren't.

I'm concerned that this is going to be a problem and would rather find out now then after I've reassembled the engine. I have emailed Performance VH but thought I'd check with the brain trust here to see if anyone has had a similar issue.

Wow--this is something I've contemplated doing on my '94---assuming that I can't bleed/restore the factory ones.
I have so many questions, and maybe not enough answers. but I'll tell you what I've found playing with old air-filled VH45DE lifters.... I think the FSM procedure gives you the recipe, but maybe not all the technique. you did have the HLAs vertical and submerged when you bled them out? What I found is that they were 'squishy' even after sitting overnight. I had to SIMULTANEOUSLY depress the checkball (vertical thru hole) and then push down on the half-dome that engages the follower. I could get a physical click and air bubbles rising when I did this.

My thinking was that if I was going to go to the trouble of removing the old ones it just made sense to replace them rather than bleeding and reinstalling the old ones.

I did follow the procedure in the FSM; depressing the check ball with a paper clip while submerged vertically in engine oil. Definitely got air bubbles out, and found the same as you, that the check ball had to be depressed and the top of the lifter had to be compressed as well in order to bleed them thoroughly. I probably should have kept track of which ones I bled and which were incompressible right out of the package that I installed without bleeding, but even after removing, re-bleeding, and reinstalling I still have some with height differences, although not as dramatic as the ones in the photo.

Performance VH has replied and asked how many I have issues with, and noted that they should not compress, which makes me think that bleeding was not supposed to be necessary. Being that the FSM states that air in the lifters will not bleed out with use, I'm thinking that any differences in installed heights at this point are not going to change once the engine is running again.

bleeding probably a good idea on all of them--no way to keep them vertical during shipment, so likely some fluid migration/air ingest in all of them. I think what I would be most concerned about is that the valve is slightly open when on the base circle of the cam lobe.
If it is, then i'd check that again in 24 hours, and see if the pressure of the valve spring has "balanced" the HLA somewhere in mid range of the half dome.
The whole idea of the HLA is to maintain zero clearance over temperature and over life. So idelally, the HLA piston will not be at either extreme and therefore allow operational adjustment in either direction.
I know you probably know all that already, just putting it out there.
Did you remove the cams to do the HLAs? Or were you able to use the Mitsu 3.0L V6 tool (I posted a question in the VH engine forum a while back on this) that compresses the valve keeper to allow removal of the follower.
Do the pVH replacements come with the 1mm o-ring near the top of the HLA?
Did you compare the oiler hole on your factory '90 HLAs with the pVH replacements? I believe the later engines (maybe from around '92, not sure) got a revised HLA with a much larger oiler hole in the side. Wondering if the pVH units have the larger oilers?

I'm not sure that the lifters are capable of being extended far enough to keep the valves open at all - my only concern in that area would be that somehow they might not be opening the valves enough, and the valve lift would suffer. I'd like to think that when they pump up with oil pressure they are fully opening the valves, so that wouldn't happen.

I postulated to PVH the idea that there was no way the lifters remained vertical during shipping, so bleeding should probably be a recommended procedure for every lifter prior to installation. As it is there were no instructions included with the lifters, nor is there anything on their website.

Funny you should mention that Mitsu V6 valve spring tool as that's exactly what I'm using, leftover from replacing the lifters on my 95 Montero with the 3.5 V6. With some creative manipulation and even more creative language I was able to get every one of the valve springs compressed to remove the cam follower.

The PVH replacements do not have an o-ring. I'd like to think they've been tested enough to make sure that it's unnecessary.

As near as I can tell the oil holes in the tip of the lifters are the same size, but I don't think that's the oil supply for the lifter. I didn't compare the sizes of the holes on the sides of the lifters, but I will with the next one I remove now that you've mentioned it.

Well, if you have a 'hyperextended' HLA next to a 'normal' one (on the same cyl either intake or exhaust), then you would be able to visually see a difference in the valve travel, when the cam lobe is at the peak?
Thanks for the notes! One of my hesitancies in doing this job has been (lack of inspiration for) removing the cams. Of course, dropping a valve would be a really bad day. Might be a good idea to only work on cylinders when the piston is at the top of the bore. Anyway, knowing you were able to use that tool is great info! Your engine looks pretty clean--were the old ones easy to remove, and how many were 'collapsed'?
You're right, the oil supply (what I refer to as the oiler), is on the side of the HLA.
ANd, yeah I've always wondered why nissan put those o-rings from the factory when almost all other engines don't have them. My hypothesis is that they were worried about oil supply passing between the HLA body and the bore, as opposed to having to go thru the HLA.

Is it even possible to have a lifter installed that prevents the valve from closing completely? I've never seen that being possible on any other engine I've worked on, but this is my first VH45DE. I'm asking because I'm getting some odd readings when I do a compression test - cylinder 2 for example tested at 0 psi initially, then 100 psi, then 0 again. I'm not entirely sure how this could happen so I'm borrowing another compression tester just to confirm that the issue isn't with the gauge.

The old lifters were not difficult to remove, the only issue was being able to get the valve spring compressor tool in at a workable angle while trying to work around the brake booster, ABS unit, and a/c lines. All of the old lifters were 'spongy' to a point, some more so than others, but none that I would consider to be collapsed and non-functional.

So the variations in lifter height were completely due to them not having been bled - I removed and bled all 32 of them and they are all at full height now.

I also compared a new lifter to one of the old lifters after bleeding them both and the difference in height was negligible. The oil supply holes in the sides of the lifters were larger on the new replacements, but the holes in the tips were slightly smaller.

good to hear! How's she runnin'?

Not quite back to running yet. I think I finally have the cam timing set properly and now need to get the valve covers and plenum reinstalled.