Penalty for a fractious P-motor: Off with its head(s)!

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Tom Johnson
Tom Johnson

I had two reasons for taking the heads off my P-motor. For starters, oil leaks had sprung up around the front head- a big one at the front of the dishpan, the other between the head and cylinder where oil drains from the head, through the cylinder, into the crankcase. The rear head was leaking around the drain too. The other problem was more serious. The P stalled after the last cam swap, and nothing I tried got it going again. Out of ideas after confirming that the engine had spark at the right time, gas, and at least fair compression, I swapped out the cam, ignition switch, module, coil and wires, and checked the pushrods. I also wired the battery directly to the coil to rule out wild cards like a faulty replacement switch and intermittent shorts in the wiring.

I might as well have faced the North Pole and prayed to Rudolph the Red Nosed Reindeer for all the good it did. At that point, I started wondering if a manufacturing glitch with the cam had caused the valves to nick each other, dinging the edges enough to keep them from sealing. The P had compression, but the kick didn’t feel as stout as before, so valve problems seemed a possibility, as did a blown head gasket- although the gaskets in P-motors are copper and pretty tough. Checking compression with a screw-in gauge, I found the numbers to be 20% lower than last time, although that seemed inconclusive because I could only check them with the engine cold. Considering the oil leaks and that I couldn’t find the reason my favorite motorcycle no longer ran, off came its heads. Thanks to the 12-point head bolts S&S uses, getting them off was a breeze. I broke the bolts loose with a ground- down 7/16” box-end wrench, then spun them out with a socket on the end of a ex-extension. Removing the ignition- timer head took only a few seconds, and made reaching the head bolt that hides behind it vastly easier.

The head gaskets were intact and showed no sign of a compression leak, although an oxidation pattern indicated that clamping force had followed the head bolts pretty closely, leaving much of the gasket surface open to air- and oil leaks around the drains. Compression sealing in Pans and Shovels occurs at the fire rings, the 0.200” lips that surround the bores on top of the cylinders. Fire rings are great unless the cylinder or head is machined wrong, making the re ring too tall. In that case, the head can’t compress the gasket enough to seal it. To rule that out, I placed the heads on the cylinders to measure the gap between them with a feeler gauge. More than .035”, the thickness of the head gaskets, would mean that the heads couldn’t seal the gaskets, but there was no gap at all. A more common problem in heads with head-bolt inserts is that the inserts can pull out far enough to keep the gaskets from sealing. A quick glance showed that the inserts were where they belonged, and a sweep with a steel rule confirmed that all gasket surfaces were at. The next step, watching the pistons move through their travel as I turned the engine over, revealed even wear patterns in both cylinders, with no evidence of blow-by or any other problem with the rings or pistons.

Next came the valves and seats. Using a simple trick that probably goes back to the birth of the four-cycle engine, I filled the combustion chambers with gasoline, let them sit for a few minutes, carefully poured the gasoline out to avoid getting any inside the ports, and took a look inside with a flashlight. There was a hint of leakage around the front intake valve and rear exhaust- not a lot, but maybe enough to matter. (Safety warning about gasoline: Don’t breathe the vapors, put it anywhere close to an open flame, or otherwise try to blow yourself up or light yourself on re. It happens.) Another observation was that both pistons had a lot of oily carbon build- up for an engine with only 10,000 miles on it, as did the combustion chambers. Because the bores looked good, I decided to leave the cylinders and pistons alone but break the heads down for a closer look. Watching for shims as I removed the springs, I found one under the lower collar for the rear exhaust valve. Because shims are used to set Installed Height (more on that later), their locations are important if you don’t alter the valves or seats and plan to re-use the springs and associated hardware. I packaged each valve and spring assembly separately in labeled freezer bags and then checked valve-guide t with a common backyard technique- I stuck each valve in its guide, sealed the top of the guide with a fingertip, and yanked the valve out. The exhausts weren’t as tight as the intakes, but all were tight enough to deliver a decent “pop.” (A weak pop, or none at all, indicates that the valve or guide is worn and needs attention.) The seals, however, which appeared to be K-Lines, were clearly shot. I hadn’t noticed any exhaust smoke, but guessed that the worn seals and sloppy guides accounted for the oily pistons and chambers. 

A seal that lasts longer than 10,000 miles would be nice, but otherwise K-Lines are ok. They’re cheap and available just about everywhere, but the best thing for me is that they’re short, which reduces clearance problems between the top collars and seals with high-lift cams. I would rather use spring-reinforced seals, as available from Kibblewhite, Rowe, and other sources, but would have had to shorten the tops of the guides to make room for them with the cams I like. Not that big a deal, except I felt that the tops of the guides were stressed too much by my cams to shave much metal off there, so K-Lines it was. Are valve seals really necessary? My P-motor uses 1/3 the oil my best seal-less Shovel did on its best day, so I say “yay” to that one. And as I said, the exhaust guides were a little sloppy and could use some help. I should probably have completely rebuilt the heads while they were off, but was in too much of a hurry to get the P back on the road. I hope not to regret that for at least two months.

Originally, I’d planned to soak the valves and combustion chambers with carb cleaner, wire brush them, lap the valves and seats with Clover compound if I found no damage, and then put everything back together to see if that, or an Act of God, had fixed the starting problem.

But just after I finished cleaning everything up, I remembered my pal Wes Brown, the proprietor of Cycle Rama in Pinellas Park, Fl. Besides having decades of experience porting heads on everything from 30.5” Triumph flattrackers to 180” Top Fuelers, Wes owns a Serdi valve machine- a big deal because Serdis don’t grind valves and seats, they cut them. And because the head that supports the pilot and cutter oats in a column of air, angles cut by Serdis are precise and beautifully finished- which translates to improved air ow. I knew all that before Wes “Serdi ed” my heads, but seeing the finished product, ready to go on my engine, said more than anything I’ve ever heard or read about Serdis. I think you’ll see what I’m talking about if you take a close look at the valve seats in Photo 5. Wes cut five seat angles with his Serdi, creating more rounded surfaces than a conventional three-angle valve job ever will. The result is that air, be it intake or exhaust, can start moving quicker and then move faster. If you’re interested in a topnotch valve or porting job, call Wes at 772-546-0889. He was taking a phone order from Australia when I walked in, so mail-order shouldn’t be a problem. Neither are non-Harleys- Cycle Rama is not brand-specific.

The first step in reassembling my heads and valves was to check the Installed Height of the valve springs- the spring assembly must be compressed to the dimension specified by the manufacturer to provide the correct tension and accommodate the advertised cam lift. My springs are from S&S’s .590- lift kit for Shovels, and require an IH of 1.480”, +/- .020”. Other checks such as tension, coil bind, and clearance between the spring assembly and cylinder head/ rocker box, are part of setting the springs up. I skipped them, except for a quick glance here and there, since everything had come out of a relatively low-mileage engine that had never been over-revved- the best beatdown for valve springs there is, next to overheating.

But Installed Height was a different story since the seats and valves had been cut, sinking the valves deeper into the heads and increasing the IH of the springs. The easiest way to check IH is with an Installed Height Micrometer, as available from Isky Cams and performance auto-parts distributors like Summit and Jeg’s. At 40 bucks or thereabouts, height mikes are cheaper than they sound. Mine was too long for the P heads, though, so I used a divider and caliper instead, as covered in the cam article a few issues back. I started by installing a valve in its guide, placing the bottom collar on the head, followed by the inner spring from the spring pack to stabilize the collars, and then installing the top collar and keepers. After giving the tip of the valve stem a smack with a plastic hammer to seat the keepers, I checked the distance between the collars with a divider, as shown in photo 6, and measured the divider span with a caliper. I took several different measurements after rotating the top collar a few times to be sure evil spirits weren’t forking with me. Compared to increasing IH, which requires machining of the head or one of the collars (if condoned by the manufacturer), decreasing IH is a breeze, even for us amateurs, requiring only that shims be placed between the bottom collar and head. S&S specifies an IH of 1.480” for the .590 springs, +/- .020”. Using my front intake as an example, the first IH measurement was 1.527”, .027” outside spec. No problem- I stuck a .030” shim under the bottom collar to compress the spring and decrease IH to an acceptable 1.497”, then measured it again to con rm. I could have added a .015” shim to lower the IH to 1.482”, almost perfect, but didn’t- the reason being that setting the IH up on the high side of the recommended specs created a little more room for valve lift, at the expense of a slight decrease in spring pressure.

The next step was to measure clearance between the valve-guide seals and top collars. There should have been more clearance than before, if anything, but I wanted to be sure. I started by installing the shim, bottom collar, and valve. You can use Scotch tape in a pinch, if you get the valve stems clean enough for it to stick, but otherwise a plastic sleeve (provided with some seal kits) should go over the tip of the valve stem to protect the seal as you install it, which comes next. Some go on with finger pressure, but K-lines require an installation tool of some sort and a few taps with a small coercion tool, as shown in Photo 7. With the first valve and seal in place, I installed the top collar and keepers, then gave the collar a good yank to seat it against the keepers. Next, I pulled the valve all the way up, held the keeper square, and measured the distance between it and the seal with a divider. The minimum distance is valve lift + .060”. My smallest clearance was .712”, so I was in fat city with the .591 lift my 640 cam delivered with 1.5:1 Shovel rocker arms (compared to the 1.625 Evo rockers the cam was designed for). With insufficient clearance, you’d need to remove material from the top of the guide or the bottom of the top collar, if recommended by the manufacturer. It took a while for me to learn this, but buying a few extra seals, a whole ‘nother set, even, is cheap insurance for getting cylinder heads back together on schedule.

Now for the oil leaks. S&S has changed pan gaskets since I bought my engine, but I had already ordered James’s coated metal gaskets, James #JGI-1754-48-DL, when I learned that, and stuck with them. I’m usually pretty good about following manufacturers’ instructions, but went my own way this time because of the leaks. The pan gaskets leaked mostly around the pockets for the exhaust valves, because that’s the lowest area of the head and oil tends to pool there- as pointed out by another pal, Al Schumann at S&S. I started by placing the new gaskets on the heads to see how they t. The screw holes were fine, but the areas nearest the valve springs would have rubbed; I trimmed the trouble spots with a metal fingernail file, shooting for a clearance of .080”. James recommends installing the rubber-coated gaskets dry, but I applied a thin lm of high-temp silicone to both sides of the problem areas around the exhaust valves. Sealants can serve as a lubricant and let gaskets slip out of place as the hardware is tightened, but not this time. After wiping down the pan screws with lacquer thinner and applying a drop of blue Loctite, I started the torque sequence with the two screws closest to the exhaust valve and then followed an X-pattern, working towards the center and tightening the screws gradually, in small increments. S&S calls for 4 to 6 in-lbs. of torque, but none of my torque wrenches go there so I took my best guess and tightened the screws by feel- I stopped tightening before it felt like something was going to strip.

I understand that S&S uses Gasgacinch to seal copper head gaskets. Considering my leaks, I decided to try something different. After cleaning the gasket surfaces on the heads and cylinders, wiping them down with lacquer thinner and masking off the drains (being careful to leave enough room around them to keep sealant from oozing in), I sprayed the bottom of the gaskets and the gasket surfaces of the cylinders with Permatex Copper Spray-A-Gasket and placed the gaskets on the cylinders.

Then I put a thin film of Permatex Ultra-Copper, a high-temp, copper- impregnated silicone compound, on top of the gaskets and laid a slightly thicker bead around the drains, again taking care to avoid getting too close. Since the head gaskets leaked only around the drains, I followed a torque pattern used by a long- gone brother named Tree. Working in small increments again, I tightened the two bolts closest to the drains and then followed another X-pattern, ending with the last bolt on the drive side- the one closest to the center of the engine. I can’t really recommend any of this since I’ve never tried it before and don’t know how well it will work, but it makes sense to me, especially Tress’s torque sequence. One thing I can recommend is to steer clear of the old Pan/Shovel routine. Pulling the head bolts as tight as you can get them with a two-foot extension and then repeating the same routine after the engine’s hot will only pull out the threaded inserts S&S uses in head- bolt holes. They look bulletproof, but the inserts are just thread reducers that allow smaller-diameter head bolts, which permit more beef in the cylinders around the bolt holes. Do lube the threads and the area between the head-bolt angles and washers with anti-seize according to the instructions, then tighten the bolts to the recommended 50 ft-lbs. and leave them alone. Don’t keep going round and round, trying to get them tighter and tighter, and don’t re-torque them later. You will eventually pull an insert out- a dead giveaway that you ignored S&S’s installation instructions and voided your warranty.

Surprise ending: It took a squirt of ether to start the P after it was back together. It only ran for a few seconds before dying, and was sneezing like crazy the whole time- a dead giveaway of a lean fuel mixture that led to an interesting discovery. The inlet valve in the Mikuni was jammed solid with corrosion, as was the idle/pilot jet. After I replaced the pilot jet and cleaned the inlet valve, the P red right up and ran the same as it ever had. Back when all this started, I’d checked for fuel with the accelerator pump, which worked fine since there was already fuel in the oat bowl. It hadn’t been able to get into the carb or engine though, and that’s what counts. Live and learn, I guess, but at least the P has a fresh valve job and the oil leaks are fixed…I hope.

Tom Johnson
Tom Johnson

Many thanks to Al at S&S, Wes at Cycle Rama, and Mark at Daytona Black Gold (541 Ballough Rd., just around the corner from the BMW shop west of the Main Street Bridge). No matter how obscure the two-bit parts I come in looking for, Mark always has them in stock.

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