Learn me: Shim-under-bucket valvetrain (Mazda BP content)

If this was old-school pushrod V8s, I would know the answer, but overhead cam stuff I'm a little unfamiliar with. I have a 2000 Mazda BP-4W with the solid lifter shim-under-bucket valvetrain. Are the shims just to make the height of the valve stems uniform? Or are they to set the valve lash on each valve stem/cam lobe, like setting valve lash at a rocker arm? The reason I ask is because I sent out my engine to a machine shop and got everything redone and am now considering dropping in the BP-5A intake camshaft and I want to know can I just bolt it in or am I going to have to have all the lifters re-shimmed?

They're to set the lash.

I'm not a Mazda guy. But, in Lotus and Toyota circles, the shim is what sets the valve lash, or clearance. You have to put the whole thing together with known value shims, then measure your lash, then do the math and figure what the shim thickness should be based on what you measure and what shim is in there, then look at your shim inventory and order the ones you don't have, then take the cams out, pull the buckets, put the right shims in and put it all back together.

This is even more phun on a Lotus 910 motor with the cams riding in a removable Cam Tower that you have to take off to get to the buckets. In that case, you also have to take the thickness of the form-a-gasket material you will be using under the cam tower into account. Your Mazda motor won't have that problem.

You're going to have to re-shim the lifters, but it's easy to DIY, all you need is a feeler gauge. You may get lucky and not need all that much, when I swapped out a damaged BP4W exhaust cam for one out of donor car, I only had to change one or two shims.

Shims are $5 each if you order through Mazdaspeed, quite a bit more at the dealer. If you know any local Miata guys see if they have a pool of spare shims.

Alright, thanks all. Like I said, if it was an old pushrod engine I would've known the answer. If I had known the engine was going to be salvageable, I would have ordered the cam and sent it with the engine and had the shop set it up, because he reshimmed all the lifters while it was there. But I wasn't sure, and he never contacted me during the process, I had to call him to find out it was done in fact, so I didn't know, I did at least get Mazda Motorsports 0.10" over pistons in the process. Judging by the fact that no one said otherwise, I'm going to assume that the BP-5A cam isn't snake oil.

It's not snake oil, but it's not a huge bump either. It's still a stock cam in Japan, so it's not super lumpy, maybe worth 5ish hp at the top end, a bit more if you've got boost.

In reply to codrus:

Well, my thought process is this, and feel free to alert me if this sounds to cheaty: I run the car in C/SP, and the rules for C/SP state the camshaft must be a standard item. The BP-5A was pretty much the same aside from the cam and intake manifold, and C/SP rules allow changing the intake manifold. So couldn't I declare my engine as a BP-5A and run the cam and then say I swapped to either the BP-4W intake manifold/Honda intake/ITBs? Note: I am not running at a National level.

Also, even if it's only worth 5ish hp, I'm also running a RB header, which is supposedly 5-10hp, and that should put me at a good bit more power than the stock 1.6L (with a better powerband). And I drive the car 3 seasons out of the year on the street and to and from events and on long roadtrips, so I don't want to deal with a grumpy, lumpy cam.

Sounds cheaty to me, thanks to this sentence "The updating and/or backdating of engines, transmissions, transaxles, and/or unibodies must be done as a unit; component parts and specifications of these units may not be interchanged"

Aren't BP5As EU/JDM or Mazdaspeed only? Either way, you can't swap one in. Gotta be a US configuration, and the MSM is in BSP.

Practically speaking? Probably makes no difference.

In reply to cmcgregor:

Yeah, makes sense. I am literally the only SP car in my chapter, so I doubt I'd get protested. I just didn't want to do blatant things like run a set of 292s and tell people it was stock as it sits there loping away

cmcgregor wrote: Sounds cheaty to me, thanks to this sentence "The updating and/or backdating of engines, transmissions, transaxles, and/or unibodies must be done as a unit; component parts and specifications of these units may not be interchanged" Aren't BP5As EU/JDM or Mazdaspeed only? Either way, you can't swap one in. Gotta be a US configuration, and the MSM is in BSP. Practically speaking? Probably makes no difference.

AIUI, this is correct. The BP5A cam will pass smog fine, won't set any codes in the ECU, and will sound like a normal cam, but it's not legal in a CSP Miata. Probably nobody will care at a local event because the difference is so subtle, but a nationals-level event is a different story. Also, if one were inclined to cheat, it's pretty obvious on a teardown because the part number is cast right into the cam.

In reply to codrus:

Yeah, no smog where I live. And no intentions of ever going to Nationals. And even locally, because my car is underprepped by a long ways, I run mid-pack raw, which equates to bottom of the heap on PAX. We also had a season once, where every STS car wasn't actually STS legal.