Head Rebuild

A compression test on the engine revealed low compression in the #2 cylinder (80 psi), and the other cylinders ranged from 117 to 130 psi. A leakdown test in the #2 cylinder revealed that air was leaking into the #1 cylinder, through the rear vent vapor tubes, and from the dipstick (70% leakdown rate). There was oil in the coolant (but no coolant in the oil), and all of the valve clearances were tight, particularly the exhaust valves. The #2 exhaust valve had less than .002" clearance, possibly negative. So I made the decision to pull the passenger side head, to see what I could find, expecting a blown head gasket.

Upon removing the head, I found a crack in the #2 exhaust valve (red arrow). This was most likely caused by the inadequate valve clearances (e.g. improper maintenance). All of the exhaust valves appeared to have burned away at the edges, reducing the valve margin by about half. You can barely see this in the photo -- there is a concentric line showing on the outside edge of the valve, but mainly at the 1 through 6 o'clock positions.

The head gasket was a bit crumbly in places, but it was mostly intact. This photo shows how the head gasket facing has broken away and has exposed the inner steel core. (It's possible that this was caused during the removal of the gasket from the head/blocks, as it was slightly stuck). The hole near the arrow seals the pressurized oil that goes up into the head and the camboxes.

There was some electrolysis on the passenger side head and block face, but it wasn't serious enough to warrant replacement of the head or block. The damage was located near the top middle stud, away from the coolant passages and the combustion chambers.

I began disassembling the head myself, rather than rely on the machine shop, because I wanted to thoroughly inspect all of the parts and the head. Also, I wanted to double-check the shop's work after I got back the assembled heads.

This valve spring compressor is especially suited for the Biturbo engine. I went through three different valve spring compressors before I bought this factory tool from MIE. The high walls of the cylinder head and their distance to the valve springs makes it difficult to use an ordinary C-type compressor. The valve stem locks were stuck to the valve, so I had to hammer on the valve head with the blunt end of a chisel to free them.

After removing the valves, the valve stem to guide clearance can be checked by putting a dial gauge on the valve as shown and wiggling the valve in its guide. All of the guides on the head were excessively worn; the exhaust side more so than the intake. I could see and feel the valves wiggling in the guide. At this point, I pulled the other head, figuring that its valve guides were also worn.

If it hasn't been done already, the oil restrictor piece in both heads should be removed and the brass filters inside blown out. The restrictor piece can be re-installed when the heads are returned from the machine shop for cleaning.

I sent the heads and the associated hardware to a machine shop for a complete valve job. This included checking the heads for cracks, resurfacing, installation of new valve guides and valve seals, replacement of all the exhaust studs, and grinding of the valves and seats. I gave them a copy of the factory service manual dealing with the rebuild of the heads. I also gave them a marked-up copy of the AERA "standards of service" for cylinder head rebuilding as a means of telling them what kind of work I wanted done, and how I wanted it done. The heads were returned in three weeks.

Head Rebuild -- New Parts

The original exhaust valves were a two-piece design (on left). They were all replaced with one-piece stainless steel exhaust valves from MIE (on right). The intake valves were in good condition, so they were reused. All hardware, both new and used, should be throroughly inspected before it is installed. In particular, the valve margin (red arrows) on the original valves should be closely examined to check for any signs of burning.

The width of the slot which holds the valve keepers (blue arrows) has been enlarged from 2mm to 3mm. This changeover took place during the early US production. When replacing the older valves with new ones, the valve spring retainer and the keepers need to be changed. The new hardware has different part numbers from the old. The valves in my '86 car already had the 3mm slot, but I bought some new keepers and the retainer. For some odd reason, the retainer that I received didn't fit the same as on my original valves -- it sat higher on the valve, even when combined with the original valves and keepers. So I reused the old keepers and retainer and discarded the new ones.

Head Rebuild -- Cleaning and Preparation for Reassembly

While the heads were at the shop, I began cleaning the cylinder head studs, block face, and tops of the cylinder liners. Before wire-brushing the studs, I fashioned some cardboard to cover the block and liners so that crud would not fall into them. The studs had quite a bit of rust and corrosion on them.

There might be some corrosion buildup in the coolant passages of the block, down near the base of the cylinder liners. This buildup may inhibit heat transfer between the coolant and block. This corrosion can be carefully loosened with a long narrow screwdriver and vaccumed out. Don't blow it out with compressed air because it will go everywhere and you won't get all of it out.

I wiped off the excess oil and crud from the block and liners using brake cleaner applied to a lint-free towel, and then cleaned the block face and liner tops using a red Scotch-brite disc. These are available from the Eastwood Company (among other sources) and do a very good job of cleaning aluminum and iron surfaces. The discs attach to a drill and leave a very fine scratch finish on the surface. The drill does not need to be turned very fast in order to leave a clean finish. A machine shop owner told me that the discs will round down corners if you're not careful.

After finishing with the disc, I did a final wipedown with brake cleaner on the block face and tops of the cylinder liners. I sprayed WD40 on the liner tops and cylinder walls to prevent rust formation. (Be sure to clean off the WD40 from the liner tops when you install the heads). After cleaning, I checked the block face for flatness using a precision straightedge, and checked the cylinder liner standout.

I checked the newly installed guides after I got the heads back from the machine shop. This time I bought a small-hole gauge made by Starrett to measure the inside diameter of the valve guides (photo). This tool also allowed me to measure the stem to valve guide clearance fairly accurately (within about .001"), which was close enough to check the shop's work. McMaster-Carr Supply company sells this tool for about $20; the part number is 829B.

When the machine shop mills the heads, there might be some leftover flash on all of the edges of the cylinder head face. You can mostly feel it with your finger tips, and sometimes see it with the correct lighting. This flash can form a hot spot in the combustion chamber and cause pre-ignition. It can be removed with careful sanding using emery cloth.

All bolt threads in both heads should be cleaned or inspected thoroughly, including the bolt holes for the coolant housings on the rear of the heads.

Head Rebuild -- Assembly

The OEM head gasket is manufactured by Goetze in Germany (pronounced Gert-za), and is available from both MIE and AW Imports. Before installing the head gasket, the exhaust manifold should be checked to see that it will bolt on correctly to the heads. I found the the driver's side manifold didn't want to bolt on, and I had to grind off some material from the exhaust manifold bolt holes.

The intake and spark plug ports should be blocked to prevent debris from falling into them. The service manual recommends using new head nuts and washers and a light coating of oil on them before torquing down the heads.

Getting the exhaust manifold nuts back on is the hardest part of the assembly job. There's not much room to work with and the nuts tend to rub against the manifold, making it difficult to get them on correctly. In one case, I had to use a nut without a flange. Anti-seize should be applied on the nut threads before installation.

I used a combination of open-ended wrenches, box-end wrenches, and a flex socket wrench with extension. A magnetic pickup tool is helpful for guiding the nuts into position on the studs.

With the heads installed, the coolant housings on the rear of the heads can be installed. The passenger side coolant housing requires some sealant on the metal plate where it faces the housing. I used some Ultra-copper sealant on it. The coolant housings take a bolt with a special washer that has a rubber gasket on the inside. Always replace these washers.

The rest of the assembly process is pretty straightforward. I put some assembly lube on the cams and buckets.

I had some serious problems with the engine after I got the heads re-installed. When I first started cranking the engine, and before I started it, I noticed that the coolant reservoir was being pressurized. I pressurized one of the cylinders with air and I could hear water bubbles inside the block. The other five cylinders did not produce the sounds of water bubbles. After removing the cylinder head, I found coolant in all three cylinders. The other head had the same problem.

With help from other Maserati owners, I found that the machine shop had milled the heads with a surface that was too rough (see photo), with the result that the cylinder head wasn't sealing properly. I discovered a tool called a surface comparator gauge which can be used to measure the average roughness of a milled or ground surface. It is a plate of metal with several strips that have been milled or ground to various roughnesses. By comparing the strips against a workpiece, the average roughness of the piece can be estimated.

These tools can be found at McMaster-Carr, although they are a little expensive. Supposedly you can special order a $30 one from Pep-Boys. It's made by Fel-Pro; the part number is SC-1. Specialty tool shops should have these also; call the manufacturer to find a distributor in your area.

I walked into one tool shop and managed to get a look at a surface comparator, and I saw that my heads had been milled to an average roughness of greater than 125 microinches. I found several recommendations for a range of 50 to 60 microinches for aluminum heads. I took the heads to another machine shop and had them resurface the heads to the desired roughness. They used a grinder instead of a milling machine. The heads were re-installed and the problem was fixed. The second photo shows the surface of the head after the second shop had milled it.

In the 8/90 issue of Road&Track magazine, a Maserati service shop owner says that the valve clearances on the Biturbo tend to tighten up faster than they otherwise would if the car didn't have so much underhood heat. This heat comes primarily from the exhaust headers, the three catalytic convertors, and the turbochargers.

I am planning to conduct experiments to see if the amount of heat is excessive, and whether the following modifications will reduce it and by how much:

• Coating the inside and outside of the exhaust headers and the turbo downpipe with ceramic material. One vendor that offers this service is Jet-Hot.
• Removal of the catalytic convertors and the air pump (off-road use only).
• Cutting vents in the top of the hood and installing an oil cooler. I originally saw these ideas in a Tech Tip article in the VCM magazine from MIE.

Head Rebuild -- Epilogue

Since the rebuild, I've put over 500 miles on the car and it's been running great, with no oil appearing in the coolant. I sure wish I could have pinpointed the exact cause of the oil getting into the coolant though