Issue #227 Cautionary Tale | Valves, Head, Valve, Had, Pockets

You are here: Home

Issue #227 Cautionary Tale

Written by Robert Smith
Wednesday, 25 April 2007
There’s a simple principle in scientific experimentation: change only one variable at a time. That way, if you get a new or different result, you know (in theory, anyway) what caused it. The same is true in tuning motorcycle engines. A friend called me to look at a Commando engine he was assembling. He’d tried fitting the cylinder head, but was unable to get enough adjustment on the intake rockers to set the valve clearance. And with the head snugged down, it was pretty obvious the intake valves were hitting the pistons. Something definitely wasn’t right. We tried to trace the engine’s pedigree. The bottom end had definitely started life as a Combat model: the engine number was in the right range (roughly 201000-211000), and the engine breather was located at the rear of the crankcase. As most Combat engines suffered accelerated main bearing failure, it was a reasonable assumption the bottom end had been apart at some time. But the rest of the motor didn’t quite add up. Combat cylinder heads are stamped “C” on the top between the exhaust rockers; we couldn’t find any such mark. And the intake ports were 30mm diameter, not the Combat spec 32mm. So this probably wasn’t the original head. Not only that, but the pistons were the dreaded slotted type—a wide oil slot under the oil control ring. These pistons worked fine in the Atlas, but when fitted in the more powerful Commando engine (especially the tuned Combat motor), the crown would sometimes separate from the body. Not good. We tried to find out more about the motor. The owner claimed it had been running fine before he took it apart, but also owned up that the cylinder head had been worked on since, and machined to increase compression. Knowing this, we tried assembling the engine with a compression plate under the barrels. Still no good: the intake valves were still hitting the pistons. So we took another look at the head. As well as the machining work, it looked like larger valves may have been fitted. A quick check with the calipers … and yes, the intake valve heads measured 1.56” instead of the stock 1.49”. And that should have been the answer. Commando pistons are essentially flat-topped but have valve pockets machined into them. Around top dead centre on the exhaust/intake strokes, both valves are open at the same time, the intake gasses helping to push exhaust gasses out. This valve “overlap” means the angled valves in a Commando head would hit the pistons if the pockets weren’t there. But the pockets are only large enough for the standard valves. The larger valves in our modified head would need larger pockets. Deeper pockets, too, but this time pants pockets. In went a pair of Australian made JP pistons with bigger pockets, together with the compression plate. Problem fixed? Sadly, no. One of the exhaust valves was now hanging up, and with the engine fully assembled, it still wouldn’t turn over. So back to basics. If we now had enough room for the valves, and yet we still had metal-to-metal contact going on, the problem must be somewhere else. Off came the cylinder head and barrels. With a flashlight we were just able to see the markings on the camshaft. Instead of the “2S” stamp of a stock Combat bump stick, the camshaft was stamped “SS.” Of course, someone at Norton’s Andover factory may have been having a bad day and picked up the wrong stamp, but US camshaft maker Megacycle had also produced an SS cam for the Commando in the past. So, maybe it was a combination of the big valves and a sporty cam with, no doubt, more overlap and higher lift. One possible outcome: valve interference. DISASSEMBLING THE CYLINDER HEAD SHOWED THAT ONE OF THE EXHAUST VALVES WAS BENT, EXPLAINING why it had been hanging up. But more significantly, new seats had also been fitted in the head. These sat proud of the combustion chamber surface, pushing the valves much further into the combustion chamber. So, even if the camshaft lift was no greater than standard, the valves would still move further into the combustion chamber, making them more likely to clash or collide with the piston crown. It seemed like the Commando’s hemispherical combustion chambers with their wide (by modern standards) valve angle, the supposed SS cam, the bigger intake valves and the more prominent valve seats were incompatible—even though the piston valve pockets were now large enough to accept the oversize valves. At this stage, the choices for fixing the problem were falling into place. We could either replace the cam or some more work was needed on the head. The latter looked the easier option, so a local machine shop was hired to re-cut the valve seats, settling the valves deeper in the head, and therefore reducing the possibility of high lift or overlap causing a collision. Did it work? The head is still being worked on, but we’re optimistic. Watch this space … Comments (0) Subscribe to this comment's feed Write comment You must be logged in to a comment. Please register if you do not have an account yet.