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  #1  
Old 11-05-2007, 03:13 PM
redbull redbull is offline
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Default 71 iron head sportster

my iron head keeps losing the front piston. Both times it went, there was visible damage to the intake side of the piston. I only had about 1,000 miles on the last rebuild before it went. Any ideas?
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  #2  
Old 11-05-2007, 04:12 PM
Dr.Hess Dr.Hess is offline
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Default RE: 71 iron head sportster

Any damage to the intake valve? What kind of damage to the piston? Detonation, like the piston crown is blown off or melted? If so, it could be bad gas, too much compression, too much timing advance, too lean carb, intake leak, hot spot on the head, hot spot on the piston, or somthing else.
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Old 11-05-2007, 05:39 PM
Doc 1 Doc 1 is offline
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Default RE: 71 iron head sportster

What is the piston to cylinder clearance...it should be .035 to .004. Also if the female connecting rod (front) is loose from worn crank bearings and crank pin the pull on the piston will be adnormal.
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Old 11-06-2007, 11:46 AM
Kingofcubes Kingofcubes is offline
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Default RE: 71 iron head sportster

I would suspect timing, check the points gap (both front and rear) and check the advance unit for slop and wear. Does the rear plug or piston show signs of excessive heat?
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  #5  
Old 11-07-2007, 07:34 AM
redbull redbull is offline
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Default RE: 71 iron head sportster

The first time the piston went south the top of it had been blown apart on the intake side. The second time the rings on the intake side were tight in the grooves. The problem only occurs with the front cylinder. I have since put a rocker box oil gauge on the intake side of the front cylinder. On one occassion lately I noticed the gauge was reading no oil pressure to the rocker box. The oil tank level was higher than normal and the oil filter which is mounted to the front of the frame was cool to the touch. I unscrewed the filter partially and observed oil comming out. I unscrewed the oil pressure switch on the front of the pump and observed oil comming out. I unscrewed the oil pressure gauge at the rocker box and did not observed any oil leeking from the base of the gauge. After driving the bike a few miles back home the oil pressure gauge began to read good pressure once again. I opened the oil tank at this point and the oil level was back to normal. The oil filter also began to warm back up. The was no damage to the intake valve that I observed when it was taken apart the last time. Also, both pistons showed scuffing on the front and rear of them and not the sides. Is this normal?
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Old 11-07-2007, 07:39 AM
redbull redbull is offline
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Default RE: 71 iron head sportster

The piston to cylinder clearance is good. I did check the side-to-side play of the connecting rod and it was out of tolerance as stated in the manual. I was told not to really worry about the side play and that it was more important the rod had no up and down movement on the crank shaft which it does not seem to have any.
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Old 11-07-2007, 07:44 AM
redbull redbull is offline
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Default RE: 71 iron head sportster

The bike has electronic ignition unit and was timed by a harely mechanic. The rear piston showes scuffing on the front and back side only.
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  #8  
Old 11-07-2007, 01:13 PM
Kingofcubes Kingofcubes is offline
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Default RE: 71 iron head sportster

Did the top of the rods show signs of oil baking on them or excess heat, sounds like the oil supply was shutting off. Remove the cam cover and see of there is a clear path through the pinion shaft, we just tore down a 113" Bourget that had siezed the pistons 3 times and found that rust had been comming out of the frame,through the oil pump and clogged the pinion shaft and shut the oil off to the crank pin.
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Old 11-07-2007, 02:11 PM
Dr.Hess Dr.Hess is offline
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Default RE: 71 iron head sportster

I think you lost the pistons from detonation. Now, what's causing the detonation? That's the question.

The weird oil pressure sounds quite serious. I don't know how your oiling system is as I've never had to dig into that on a Sportster. Do like KoC's says, and also go through any other place the oil might be blocked off. Find a oil routing diagram for that motor and see where you have oil and where you don't and try to figure out where it is stopping.Perhaps somecompressed air or a pipe cleaner might help. I could think of a theoretical scenerio where the front oil is blocked off which could lead to a hot front head which could cause detonation which could blow your piston or pound the rings to stuck. That's theoretical, of course.

Whether or not it was timed by a Harley mechanic, I'd check it myself. A timing light is only around fifteen bucks or so. You don't need a fancy one, just the cheapest one you can find. You also don't need a clear timing hole plug thing. Just remove the plug in there and shine the timing light down the hole. Some oil mist will come out, but big deal.
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  #10  
Old 11-07-2007, 02:41 PM
redbull redbull is offline
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Default RE: 71 iron head sportster

If the crank pin opening was plugged would that cause only the front piston to blow?
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Old 11-07-2007, 02:41 PM
 
 
 
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71, connecting, davidson, diagram, diagrams, harley, head, iron, ironhead, oil, piston, play, pump, rod, sportster


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