WTF Interference BP4W
 

60K mile 99 BP4W longblock, I got it from a 3rd party, who got it from LKQ. "It's a great runner," he said. It was delivered to me without cam pulleys and damper. We replaced the various seals and gaskets, water pump, and go to put a new timing belt on it.

When a cylinder is at TDC, I am unable to rotate the exhaust cam to open exhaust valves. I get a hard stop when the lobes begin pushing lifters. If I take the crank a few degrees off TDC, I can rotate the cams without problem.

Engine appears to be untouched externally, IE not clean, intake and exhaust ports not machined, no aftermarket parts. Junkyard markings on oil pan. Pistons appear to be standard 99 pistons when looking through the plug hole with a flashlight. Unfortunately the bore scope is in the race trailer... in Ohio.

With OEM pistons, can you deck the head enough to create an interference motor?

Theoretically I think so.

One way to see if it's been decked a lot is to look at these little discs on the bottom of the head (still visible with head installed). If they're machined at all then the head has been shaved a LOT.


https://www.miataturbo.net/attachmen...ine=1433556743

Might a bent valve hit a piston?

The head would need over sized valves to do this or the head would have to have been decked probably around 20 thou or more.

take a peek down the ports with the valves open and see if you see stainless valves.

Did it with a vvt motor, think I took .060" off.

That's exactly how we ran 10.25:1 static compression (max allowed by rules) on a 99 after they took away our 10.25:1 pistons, but failed to change the compression figure.

Stupid thought but what if the valves are touching each other? Rotate the other way maybe? Or pull exhaust cam and reinstall close to TDC.

Had the same result on a different BP4W: Exhaust valves have interference with piston at TDC.

I understand the common wisdom is that these are non interference engines... I'm thinking that ended with the BP4W.

I have personally wrecked valves in a stock BP4W due to interference. Teardown revealed timing belt was installed improperly, but still...

Is this just an issue with high compression pistons? I thought I was safe with 8.6:1, even with a slightly milled head.

My last two engines both had exhaust valve interference.

First was VVT head with +1mm exhaust valves, with VVT pistons, 0.015" shaved off head.

Second was same head with FM 9:1 pistons, they too hit piston at TDC with max lift on exhaust valves.

Haven't tested a 100% stock motor, but both of mine are interference.

My 97 block / BP4W head is interference. (BP5a intake cam)
+1 valves intake/exhaust.
They will hit each other, as well as the piston at TDC.

Head is shaved .020".

If this motor was non-interference when stock, it was only so by a kitty-hair.

stretched rods from an over-rev?

Nope.

I have 11.5:1 Pistons, oversized valves, and slightly more cam lobe height than stock and there is only interference if I dial the cams too far off of straight up. My guess is you did not get the gears on the cams properly.

Bad guess.

You lack an understanding of what is actually being discussing here. Cam gear orientation has absolutely no effect on whether an engine is interference or non-interference. Re-read the thread

It's only non-interference if it stays that way when the belt snaps, leaving the valves extended and the pistons still moving.

A 100 dollar belt replacement is a lot less of a hassle than the kind that requires a new piston, new valves and about 100x more labor.

So why are we breaking timing belts and not keeping up with regular maintenance?

We're not, or at least I'm not. I made some observations when assembling engines that are contrary to popular wisdom, so I posted what I found. My assertion is that BP4W is an interference engine.

The only change to my life is I'm going with the Gates kevlar belts on my NB engines.