When it comes to affordability, Spec E30 offers some of the most cost-efficient racing on track today, especially if you can do a lot of the mechanical work yourself. One of the major costs in racing, besides the roll cage, is a proper power plant. Engine expenses in a number of other racing classes can cost many thousands of dollars to be competitive. With a maximum of 155.9 horsepower at the wheels, while respecting the 2,700-pound weight rule, the series just became more competitive for the do-it-yourselfers out there.
The BMW M20 engine is rated from the factory to produce 168 horsepower at the crank. While staying within what is allowed by NASA Spec E30 rules, getting to 155.9 horsepower at the rear wheels is really quite easy to achieve with a minimal budget.
If you ask around the paddock, a common combination of what works best in these engines is a used bottom end with around 140,000 miles on it and a fresh head. Over the years, low-mileage engines have become more difficult to find. Should you happen to locate a good project engine, all that’s left to do is replace the seals on the bottom end and put it on an engine stand while you refresh the cylinder head. That sounds relatively easy, right? Well, yes … and no.
Specific tools and equipment are required to completely rebuild a cylinder head, and they can be expensive. Unless you have these tools and know exactly how to use them, your best bet is to find a good machine shop. Some things are best left to professionals. Any machine shop with an experienced cylinder head machinist can correctly perform the rebuild to your specs. No porting or other “racecar” type modifications are allowed. You must use the specifications outlined in the repair manual for repairing or rebuilding the cylinder head, because these are the only repairs you can have done to the engine to stay compliant with the rules. Rule 184.108.40.206.6. states “No engine component may be modified in any manner not specifically permitted or authorized by the Factory Service Manual, the Bentley E30 Manual, or Factory Technical Bulletins.”
The machine shop should have a resource to find the exact specifications for this particular cylinder head, however, you also should know what they are. Relying solely on any machine shop for the specs can potentially get you into trouble. They could assume that their normal production style valve job is appropriate.
To get a more in-depth look at the process and gather more information, I took a trip to one of the most respected Spec E30 shops in Southern California, and SoCal Spec E30 series sponsor, Midnight Oil Motors in San Diego, Calif. Seth Scally, builder of various engines for multiple racing series, was willing to show me around the busy shop. Midnight Oil Motors specializes in European cars of all makes and models. Walking into Midnight Oil Motors means entering a world of performance, including a Porsche Cayman GTS5 build, E46 M3 and Porsche 911 racecars, and, not to be overlooked, rotary racecars. They also have a number of Spec E30 cars and engines for different racers, including the No. 28 car of the 2013 SoCal Series Champion, Steven Stepanian. Scally sat down and discussed with me what’s involved in rebuilding a cylinder head.
For someone rebuilding his own cylinder head, the first item to check after a thorough cleaning, is to inspect for cracks and check the head for signs of warping. Aluminum heads are soft and can warp easily from overheating, or by simply loosening the head bolts incorrectly. A straight edge along the mating surface of the head, and a feeler gauge, can quickly determine how true the surface really is. The factory only allows 0.3 mm to be removed before the head must be replaced. One thing you might not know is if the head has been machined previously. To check this, measure the distance from the mating surface to the valve cover and it should show you a height of about 125.1 mm. Anything less than 124.7mm warrants a replacement, but gasket companies make .25 mm thicker head gaskets, which allow us to use thinner heads.
Once those two critical dimensions are verified, it’s time to check the condition of the valves. If you already plan to replace all of the valves with new ones, then you can skip this step. With the rocker arms and camshaft removed, leaving the spark plugs installed, simply place the head on the bench with the combustion chamber facing up. Fill each chamber with brake or carb cleaner, and after 15 minutes, check the level of the liquid.
A chamber that is low or is empty is not sealing correctly and will need to be repaired by machining the valve seat and valve, or you can simply replace the valve. Fluid in the intake or exhaust port will be a good indicator of which valve is not sealing correctly. Anytime you replace a valve, the valve seat will need to be cut, also known as a three-angle valve job. Cuts of 15, 45 and 75 degrees need to be made into the seats. This restores the seat with a fresh mating surface and only leaves lapping of the valves left to do.
Now it’s time to check the valve guides. These can become worn and cause a loss of power. It’s a fairly easy item to check using a dial indicator. First, mark each valve so you know which cylinder it comes out of. This way you can reuse them if things check out OK. Remove the valve springs and partially remove the valve. With the dial indicator up against the head of the valve, measure the lateral play in the valve. If it wiggles more than 0.30 mm, the valve guide needs to be replaced. Do this for all 12 valve guides, intake and exhaust.
That’s the meat of rebuilding the cylinder head. Remember to install new valve stem seals when you install the valves.
What needs to see the most attention in the head is valve-spring pressure and retainers. Retainers are the most overlooked part of the head. If you cannot afford to replace the springs, then have the machine shop shim them back to the appropriate rate. Scally highly recommends replacing the springs because it can help reduce rocker arm failure.
Another good thing to replace is the camshaft. An old camshaft can have measurable wear in the lobes, which can reduce the power output potential of the engine. Probably the weakest link in the M20 engines can be found in the stock rocker arms. Old age and high rpm are the main cause for failure in these parts. A legal upgrade for these is the Ireland HD rocker arms. (Ireland Engineering part No. M20rarmHD) Once the head is back together, it’s time to install it on the block with a new head gasket and fresh head bolts. Be sure to torque the head according to the procedure outlined in the service manual, or all of that work will be for nothing!
Seth also noted that since you already have the head off the car, it is a good idea to install a new water pump, timing belt and tensioner. If the cylinder head needed to be rebuilt, it’s a pretty safe bet that that these items are near the end of their life expectancy, too. Besides, it’s cheap insurance for a healthy running engine.
Midnight Oil Motors
Beware of Imitations
Back in the 1980s, BMW cylinder heads and parts were expensive. The timing belts back then were not as stout as the manufacturer thought, so there were quite a few premature failures.
A common practice at the time was to buy a complete cylinder head from a company called AMC out of Spain. They were a more affordable alternative to having the BMW cylinder head rebuilt at the time, and it was a new cylinder head, which appealed to a lot of people. Since many of these heads were sold and installed as alternatives, it is possible that you could have this head on your car and not even know it. They are not the same, and are technically not legal for use in Spec E30. The only way to identify the BMW head is the part number cast into the head, ending with the numbers “885.” On the AMC head in the same location, “AMC” is cast into the aluminum. This is difficult to locate on the car because it is behind the intake manifold.
Though they look the same on the outside, they have slight differences, and the ports are different. In addition, the head height can be 3 to 4 mm shorter than the stock minimum head height. The exhaust ports are the same. Where they differ is in the intake ports. The runners are machined to be slightly larger than stock, and the valve guides are recessed deeper into the head, which also allows for slightly better flow. On a dyno, this head makes a little more torque than stock, but its reputation among E30 enthusiasts is poor.
Another option that is available, if you cannot recondition the cylinder head yourself, or have one done locally, is to purchase a remanufactured cylinder head from an authorized BMW dealer’s parts department.