Tuesday, February 21, 2012

One More Thing for Us Knuckleheads to Worry About

Or, to be more accurate, one more thing for those of us who are Knucklehead aficionados to worry about; but that wouldn't have fit in the title box.

One of my long time customers who goes by the name "Tom" (and I suspect it may be his real name) has had the same Knuckle for as long as anyone around here can remember. Over the years, I have had my hands on and into most of the parts of his engine, but never all at once. You see, Tom is just one of many Knucklehead owners who like to do as much of their own work as possible. Back when many of us started riding Knuckles, this was often out of necessity as much as choice, paychecks being what they were.

In any case, somewhere between the turn of the century and 2005, Tom had me order up a set of new reproduction Knucklehead tappet blocks for him. I really don't recall the specific occasion, whether it was a broken flange, or maybe it was Valentine's day and he wanted a special treat for the old gal. I ordered a set from Flathead Power.

At that point in time, Flathead Power was in no way connected with S&S - it was still "in transition" between its founder, Anders, operation in Sweden, and the acquisition by S&S. That transition period, and the unscrupulous characters involved have been gently exposed elsewhere, so I won't comment further.

Tom installed the FHP lifter blocks with a new set of lifters, and went on his way - which way consisted of putting lots of miles on his '42.

Fast forward through several years of hard riding, and Tom found himself in the need of a complete rebuild following a catastrophic parting of the ways of many of the engine parts which formerly made up his '42. The word grenade could be aptly applied.

As part of the resurrection, Tom opted to use another Flathead Power product which he had acquired, that being a set of FHP Knuck heads (also pre S&S vintage). Here again I was called into the act to provide oversize intake valves along with porting work. Knowing that the master plan called for a high lift cam (by Knuckle standards) I was very careful to get the stem protrusion on all four valves in just the right place to provide for a combination of the correct valve spring installed height and retainer to seal clearance while keeping the guide as long as possible to promote longevity. That is why what happened next left us scratching our heads.

Because of the special order 3-5/8" bore pistons which were chosen to match the 4-9/32" stroke flywheels and provide his targeted compression ratio, Tom mocked up the new engine parts with clay in the valve reliefs to be sure clearances were sufficient. Immediately a discrepancy became apparent. The rear intake valve to piston clearance was approximately .100" more than the front intake. Puzzling to be sure, but a problem...? If this had been a well used set of heads with untold numbers of past valve jobs, it would not have been too surprising, but that was not the case here. Suspecting a possible, albeit unlikely problem with the new Leineweber cam, Tom substituted his older, milder Leineweber cam. Same difference in clearance front to rear. Yet another check with a stock Knuckle cam gave the same results. Mr. Leineweber's name was thereby cleared of any crime in this case.

At this point I was starting to worry that I had gotten the stem protrusion off by .100" on one valve - not such a stretch given the human capacity for mistakes and the fact that my stem protrusion measuring device features a dial that measures .100" per revolution. I had Tom bring the heads back in so I could double check my work. To my relief and consternation, the problem was not in the stem protrusion.

My next thought was that perhaps there was a difference in the height of the machined surface that the valve guide bottoms out against from one head to the other. Some careful measurements which involved placing the head gasket surfaces on a table and dropping a steel rod through the guides proved this also was not the culprit.

I was nearly out of ideas, but there was one logical check still to make. I set up the lower end for my '46 with a degree wheel to perform what is commonly known as "degreeing a cam." Finally! By checking the opening and closing events with both a stock set of lifter blocks as well as with Tom's FHP blocks, it was obvious where the problem lay. The rear intake was opening 27 degrees later on the FHP block compared to both the stock block and to published specs for the cam (measured at .020" lift). And of course, checking the tappet lift at TDC, showed the difference between the valve to piston clearances which Tom had found with clay. The timing for both the exhausts and the front intake all matched those taken from stock lifter blocks. Now the only puzzling thing left to this story is that there was no large noticeable loss of performance when Tom installed these lifter blocks! I certainly would not have expected that.

The drawing below should help you visualize what the problem is, despite the sore lack of artistry. It is not to scale whatsoever, but shows the relationship between the tappet bore and the cam lobe centerline in a way that (I think) makes it understandable.

However, a little closer look at the figures gives a clue. Looking at the .053" lift timing, the difference between the FHP and the stock lifter block shrinks to 20 degrees on the opening side and 16 on the closing. That puts the timing on this rear intake, when used with this performance cam, pretty close to that of a stock Knuckle cam. In essence, for several years, Tom's '42 was running with a "hot" cam on the front cylinder, a hot cam for the rear exhaust, and a stock cam for the rear intake. My guess is that had he been running a stock cam, the performance loss would have been much more noticeable.

An important note. When this problem became evident, I contacted the gentleman at S&S who heads up the Flathead Power division and was pleased to find out that they had found the same issue and corrected it before they went into production, so you can rest assured that if you purchased FHP lifter blocks after S&S took over, they are machined correctly. I procured a new S&S block for Tom and he can attest that the valve to piston clearances now match front to rear.

One other note. I have no reason to think that ALL the pre S&S FHP rear lifter blocks suffer from this defect. It could very well have been just one run of the blocks made during that "transition" period. But that brings us back to the title of this post, doesn't it?

1 comment:

pat said...

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