By Jay of JAE
Chapman Report – May 1994

Further to some stuff I've seen on rebuilding Twin Cam cylinder heads. Here's the current philosophy at JAE. Any further thoughts appreciated. 

General head considerations:
The thing seems pretty reliable. Look for cracks between the spark plug hole and valve seats. This, along with corrosion of the two rear water feed holes to the head (the corrosion overlaps the head gasket compression ring causing failure) are not uncommon. You may need a magnifying glass to spot the cracks.

The exhaust side camshaft bore seems to like to get wavy and tight on the camshaft. A very _light_ align hone takes care of this. Do not believe Vizard when he says the cams should rotate several times when spun, I don't think they have enough inertia to do this even on mag-lev bearings! Keep bearing clearances on the cams to 1.5 thou " at most, otherwise you will have way too much oil in the head. 0.001" is preferable. An .080 - .100 oil restrictor on the feed to the head is standard practice on racers - it's not that necessary on street cars.

The front timing chain tensioner support pin threaded hole frequently strips. Oversizes are available. Be careful when installing it as it's easy to get it crooked.

The standard water leak points are the early press in core plugs on the top, side and rear, and the aforementioned water supply holes. Replace the press ins with NPT threaded allen headed plugs. The holes behind the core plugs are perfectly sized to tap (1/2 or 3/8 npt) without needing to be drilled. Use care when tapping the side holes as the intake runners are right there. If you are concerned about original looks put in the plugs slightly deep and knock in a plain plug over the top. If nothing else do the rear one, as if it does leak it's hard to spot and impossible to fix in situ. Ditto the plug next to the oil drain back tube hole.

The standard oil leak is from the kidney shaped hole on the front right side. It's a weirdo design as it overlaps the oval shaped hole on the gasket. This is the reason for the standard TC ooze at the head, timing cover and block junction (front right) just above the oil pressure takeoff on the block. To solve the problem weld (see below) in the hole and angle drill the feed. You can also incorporate the oil feed restrictor if so desired. If you have a hard time picturing this hold up (offer up?) a head gasket to the head and you'll see.

As a sidelight, always use a helicoil (we prefer Time or Kene serts) on the hole in the top of the timing case (1/4-20 I think) for the long front bolt, it's guaranteed to trip at some point.

Soft heads and welding:
It is entirely possible to soften a TC head while welding it. It also happens from repeated overheatings. One has to get pretty carried away though. Unless you have a really lame brained cylinder head person, I don't think you could soften one just by heating it to put in valve seats. I've seen massively welded up heads and they haven't softened. I have also seen soft heads that have had only minimal welding, but in my opinion they were overheated many times while on the car. The most obvious symptom is the depression left by a head washer and an inability to hold head torque. The head-washer-dent however is quite common and not a sure sign of softening. The need for a cam align bore is also another symptom. If you find cam cap bolts pulling out you can be pretty sure the head is toast. This is pretty rare though (1 in 100).

Welding a twin cam head is straightforward for a good experienced head welder using TIG equipment. The head doesn't need a strongback but it does need to be very clean and then preheated for 1/2 hour or so to 300 deg F for large welding jobs. Small welds, as in the oil pressure feed above or minor water jacket corrosion, don't require preheating. When done the head should be allowed to cool out of any drafts (we usually put it back in the oven). The two main problems with welding a head are the difficulty of access especially down the intake runners and spark plug holes and the need to get rid of all the gunk that has soaked into the head and then bubbles to the surface and explodes or is left behind in the weld as a porous inclusion. Remember the head has sat in hot oil and antifreeze through many cycles - it's been "seasoned" just like a frying pan (cylinder bores work like this too - it's called seating the rings). Sitting there welding a head is no fun as all these toxics just keep boiling and bubbling in your weld puddle. _Any_ crud left behind makes for a porous weld. Patience is required. As is a mask. Welding a head gets expensive quickly. It's almost impossible to give an accurate quote before hand as its very difficult to know how deep a crack is until it's been ground out. Be _certain_ to get a welder who is experienced welding cylinder heads, not just a regular welder no matter how skilled. The new heads on the market don't look exactly like the original though, so if total originality is your bag...

Valve seats:
Valve seat removal and installation is straight forward. Care is required. We have never had a valve seat fall out at JAE. I have never installed a valve seat that has fallen out, I have never softened a head while installing a seat. I have put in seats crooked, but have removed them and done it right. There is a right way to do this and then there is every other way. Valve seat installation on air cooled motors, especially poorly air cooled motors, is different. This essay is getting long enough already so here are just highlights.

Removal:
If you have the equipment the easiest way is to use a MIG welder and zip a quick bead around the ID of the seat. When it cools it contracts and the seat can be lifted out. Don't try this with a gas torch as it doesn't have sufficient heat output. I can pop out all eight seats and the head's not even warm. A TIG will do it too. The English method of drilling a hole and then splitting the seat with a punch works well on older seats but the new hard and tough materials don't like to be drilled the then don't like to split. It's still possible but be careful. The seat recesses then need to be checked for roundness. I have never seen a seat which hasn't been broached at least a little when installed. If it's only slightly broached and still round its OK to put seats in. Make sure you remove the shards of broached aluminum in the corners. You want .004 to .007 inch of press on the seats. If you don't have it, then the seats bores need to be cut oversize and larger seats fitted. This is no big deal. The seat bores after this should be mirror shiny from the cut and the bottom of the bore perfectly flat and square to the valve guide. The crucial thing you are looking for here is very good seat to head contact to ensure excellent heat transfer. A seat that is not bottomed or is crooked or doesn't have good wall contact _will_ burn or fall out or crack , or, or, or.

Installation:
We cool our valve seats in liquid nitrogen before installation. They shrink enough to simply fall in and only a light tap with a piloted thumper is necessary to ensure they are firmly in. We do warm the head (maybe 200 F) and clean all oily residues. You do need to be quick about it though and get it right the first time or do it again. I'm not an ultra pro and usually screw up about one seat in 3 heads. I simply do it over. Our head guy (Chris) is an ultra pro and he does screw up occasionally. The trick here is to examine the seat with a magnifying glass and be willing to do it over again if it's not right.

Do not put in seats that have not been cooled. The press is too much. This works on cast iron because the press is much less (.002 is typical iron number). Keep the seats in a dry ice and acetone bath at the very least. H20 ice cubes ain't enough. Heat the head a bit more in this situation -250F or so. You will need a hammer and a piloted driver that fits the seat correctly. The driver is also cooled. The seat will have to be driven into position with at most 3 firm well aimed blows. Any more and you get to do it again.

Guides:
TC guides are short which is why they wear out relatively quickly and why, unless they are properly sized, they leak - I'm sure the equations to simulate blue smoke include a path length variable! As has been noted, it is possible to install valve seals - usually however only with standard lift cams. The clearance at full lift between the bottom of the spring retainer and the top of the valve guide is the limiting element. We don't use seals. Shortening the guide is lunatic and shorter spring retainers pull through. Guides must fit their bores perfectly. Few shops spend enough time making sure the valve guide bores are OK. After installation the guides must be honed (not reamed - it's not accurate enough generally) to size. Street bronze guide clearances are: .00075 to .001" intake; .00125 to .00175 (Yes, we do measure to 1/4 thou). A bronze alloy guide, properly honed (not bell shaped) and scrupulously cleaned after sizing will easily last 30 - 40K miles without undue leakage. The water pump fails at this point, and we all know the saying... "While I'm here I might as well...".

Phew! That's a broad stroke picture. If anyone wants to know more e-mail or call and I'll be glad to drone on. --Tom