Best options for M20 head assembly

[ziazhp]

Need a replacement head assembly rather than having my current machined, etc. All valves are bent, camshaft pitted, 3 broken rocker arms, the others worn, etc.

Bimmerheads seems solid. There are other options through Turner, FCP Euro, etc but they use AMC heads which I've heard are not the greatest. Any other suggestions?

[Kershaw]

You can message ForcedFirebird on here.

[Spec_E30]

I will have a running long block available at the beginning of March if interested. The head was redone in 2014. The engine had 142 WHP on a Dynojet in November.

[digger]

You don't want an out of the box amc head

[ForcedFirebird]

You can message ForcedFirebird on here.

Thanks for the plug!

[e30davie]

Buy an m20 that you can see run then get that head overhauled procrssionally. Cost me a bit to do it this way but the end result was freat.

[ForcedFirebird]

Buy an m20 that you can see run then get that head overhauled procrssionally. Cost me a bit to do it this way but the end result was freat.

Yes. The worst part about a used 885 head is they crack, and unless it's been pressure tested, you won't know. The local machine shops charge $75 around here to do one, so I made a rig to test the heads for cracks on or off the car.

Second worst is getting a head that a monkey put together. The rocker shafts get beat into the head often (even by professional shops) and they mushroom on the ends. A proper head should have the rocker shafts slide in/out very smoothly all the way through both ends, and cam should slide in and spin easily.

After checking for cracks have it checked for straight. I don't mean the deck surface, but rather the cam/shaft bores. If the head has ever been overheated and warped significantly, the cam likes to wipe out the bearing surfaces, and the rocker shafts are nearly impossible to install by hand. So, the head should be heat-straightened prior to milling if it is warped. This is one of the most often-skipped steps. Most "Head-R-Us" shops won't have a plate to shim/heat a cam-bore head. The m20 design just simply isn't common enough in most places to have specific equipment.

Now it can go to a bead blaster. You shouldn't use aluminum oxide, it pits the cam bores too much, fine glass bead is best.

Next, it should be checked for valve guide run-out. Any guides showing excessive movement should be replaced. Many professional shops don't know what they are doing here and will either knurl/ream, or put guide sleeves in (which sleeves are acceptable), but most don't carry the BMW sizes. Sure they are "7mm", but the m20 are actually 6.98mm and many "Head-R-Us" type shops won't carry .02mm increment tooling, so at the end of the day your guides are going to be too big after they are finished with your head. So, it's just best to have the guides replaced rather than lined unless you know the machinist will do it properly.

Once the guides are sorted, a valve job is in order. This will make or break an m20 head as far as performance. Sure, as long as they get a good seal, it will run perfect. But if they do a shotty job, or use the wrong angles, you could be losing something like 10% of the flow!

Finally it should be decked on the gasket surface to a minimum total height of 124.68mm (4.909") when using the factory 885 dome-shaped pistons. That limit is not set to provide piston to valve clearance, rather piston to head. The dome shape of the pistons and hemisphere head exponentially decrease piston to head clearance as the head (or block) is shaved. If you over-shave a head/block, there's still plenty of valve/piston clearance (with stock components), but you will wipe out the rod bearings from them getting smashed when the piston hits the head.