The Underlying Reason for the Top End Rebuild
Back on October 16 while out for an easy ride on the XR1200, I blew a rear
head gasket. The gasket blew out while I was passing a car coming out of Leonardtown in southern Maryland,
resulting in an immediate loss of power. I quickly shut off the engine, pulled off to the side of the road, and checked things
out. While I could not determine a cause at the time, it was obvious that the stock HD rear head gasket had physically blown
out of its space between the rear cylinder and its head, on the side that faces the front cylinder. After
getting the bike home, I delved into the cause.
As background, the top end of this engine had previously been worked on by Zippers Performance back
in February of 2010. The camshafts had been upgraded to Zippers Redshift XR575 cams, the heads had been ported, and
the pistons had been replaced with Wiseco pistons with an increased dome displacement to keep the working compression nominally
the same as stock. (Compression with this build should be changed from the stock 10:1 to a slightly increased 10.25:1.)
Since this top engine work, I had several thousand miles of riding, including a trip touring through the northeastern states.
While the headwork and cam provided a nice performance increase with a bit more low end torque and a horsepower increase up
to about 100HP, the build was not extreme and the lower end of the engine had not been touched. From a cylinder stud
perspective, the heads had simply been unbolted, removed, and torqued back down.
That said, after several months of trouble free riding, including a trip to the Fort Washington
HD Service Department in July of 2009 to have a leaky oil tank replaced where no problem seemed evident, the head gasket let
Because Zippers Performance had worked on the heads and cylinders earlier in the year, and the blown head gasket appeared
to be a problem in that area, I worked with Zippers to rebuild the top end of the engine. Being a pretty competent mechanic
myself and having all the needed tools in my garage, I wanted to pull off the heads in my garage and finally take a good look
at the work that Zippers had previously done. Zippers offered full support in terms of providing any parts that might
be needed during the work, and of course, I could have just taken the bike up to them, if I hadn't wanted to get into the
engine myself. I removed the heads on the engine on October 18, and over the next few weeks worked through examining
and measuring the modified heads and engine work, and reassembling the top end. Except for the head gasket issue, everything
else looked normal in the engine. I chalked the problem off to a fluke, possibly a bad composite stock gasket, and put
the top end back together. In fact, as I reassembled the heads and went through the HD procedure of torquing the heads
first to 96-120 inch-lbs, and then to 13-15 ft-lbs, loosening, and re-torquing prior to the final quarter turn on each head
bolt, the studs seemed to hold the lighter torque values normally. I completed reassembly on November 11 and took the
bike out for an easy 10 mile ride. Everything seemed good at that time.
Friday, November 12th, I rode the bike to work, and in this short 3 mile ride, I noticed a knock-like sound when the
engine hit 4500 rpm. Since I'm running a Thundermax autotuning ECM on the XR, I wondered whether the fuel delivery
simply need to readjust with the rebuilt engine. I took the bike back out for a ride to Zippers later in the morning,
and started to notice the knock at about 3,000 rpm under load, during the ride. Considering I was 20 miles into a ride
at that point, and halfway to Zippers, I eased the bike on up to Zippers. Once at the shop, I stopped
to look at the engine with one of their mechanics, and sure enough, the front left head bolt of the rear cylinder had spun
completely loose again. The rear left headbolt of the front cylinder was also backed off from its full torque setting.
Putting a torque wrench on the accessible headbolts, it became clear that the cylinder studs themselves were pulling out of
the engine case.
Apparently, while the cylinder studs would hold 15 ft-lbs of torque normally, the case seems to have fatigued and
is not strong enough to keep them seated when fully torqued and the engine is operating.
Since the bike is still under warranty and the problem with the studs apparently indicates a problem with the hardness
of the case material in the center of the engine cases, I called HD of Washington to look at the possiblity of warranty repair
of the bottom end of the engine. Zippers helped me load the XR1200 into one of their vehicles, which I borrowed to carry
the motorcyle down to Washington HD. To help with the evaluation of the problem, here are some of the pictures
that I took when the head gasket originally blew out, and I disassembled the engine...
I did not get a picture of the stock head gasket protruding from the cylinder prior to disassembly, but
here is a view of the blown rear head gasket.
Here is a view of the crankcase deck surface for the rear cylinder.
Another view of the case around the cylinder studs.
So, there was nothing obviously wrong with the engine case when I disassembled the top end
after the head gasket initially blew. There was no sign of cracking or deformity in the case surface near the cylinder
studs. And, short of trying to re-torque the head bolts with a method that used a final torque value instead of a final
quarter turn like HD specifies, I would not have detected the problem with the case/cylinder studs without reassembling
and operating the engine.
Click below link to return to overview of initial top end rebuild following blown head gasket
The XR1200 Engine (unplugged) - Overview