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- Dec 10, 2008
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- 7,253
Head gasket oil leaks.
My Commando has always run a big bore motor, first a 920, then an 880 and now a 924. [an 880 bore with a longer stroke]
The big bore has meant that I have had to run a solid copper head gasket. As most people who have run them for many miles know -they tend to leak oil. Just enough to stain the top of the barrels and give a few spots on the sidecovers and your boots.
I have chased this problem for as long as I have owned the bike. I have tried all the common cures such as dental floss, silk thread and copper wire around the pushrod tubes along with every sealant known to man. Some did help the leak but sooner or later it would always come back.
A few other things I have tried included cutting a groove in the barrel and installing a copper o-ring around the bore. This really helped with compression sealing -but not the oil leak.
I also grooved the head and installed viton o-rings around the pushrod holes. This cured the problem for 5 or 6000 miles but by then the o-ring would be petrified from the heat and the leak would return.
One thing I did notice on the long multiple day rides across the country was if I retorqued the head first thing in the morning I could ride all day with zero leaks. The second or third 5 or 600 mile day, the leaks would return.
So how to keep the head bolts perfectly torqued.
I tried torquing a head to an iron barrel without a head gasket and then putting it in the oven at 450 degrees for a few hours and then letting it cool. The head bolts would always loose torque.
Why? Obviously the head gasket was not compressing, since there was none.
Measuring the bolts showed they had not been stretched beyond yield.
What was obvious was that the bolt holes in the head had gotten smaller and pinched the bolt showing that the tensile strength of the bolt was higher than the compressive strength of the aluminum under the washer.
So then came .100" thick hardened ARP washers. That helped but the head still lost height around the bolt holes and the holes kept getting smaller.
Then I tried the insert type washers from ARP. They kept the holes from getting smaller but the head still lost height around the bolt holes and the head bolts still lost torque.
Then I tried special high strength bolts and immediately noticed the problem became worse.
Wrong direction...
A little over a year ago I tried something new. I put the head bolts in a mill and cut the diameter of the shank down to just under the base diameter of the threads.
I reduced the shank diameter by .062" from just above the threads to just under the head of the bolt. I used a 3/16th radius on the inner corners to eliminate stress risers. Slowly cutting the bolt in the rotating 4th axis using the side of an end mill allowed me to cut mainly in the length-wise direction.
This has turned out to be the answer. The additional stretch available from the undercut bolts has stopped the bolts from coming loose and I have had no leakage now for the past 12,000 miles -without a retorque.
So what I am using now is.
1. a copper o-ring in a groove around the chamber. It really helps prevent combustion leaks into the pushrod tubes.
2. A thin coating of liquid Teflon pipe thread sealant around the push rod tubes and oil return hole. It stays soft and flexible even at 500 degrees where all the other sealants turned to charred powder.
3. .080 thick X 3/4 OD. stainless Belville spring washers under the two front 5/16 nuts.
4. Five undercut head bolts with .100 X 3/4 inch hardened stainless washers.
I am now calling this problem cured. Jim
My Commando has always run a big bore motor, first a 920, then an 880 and now a 924. [an 880 bore with a longer stroke]
The big bore has meant that I have had to run a solid copper head gasket. As most people who have run them for many miles know -they tend to leak oil. Just enough to stain the top of the barrels and give a few spots on the sidecovers and your boots.
I have chased this problem for as long as I have owned the bike. I have tried all the common cures such as dental floss, silk thread and copper wire around the pushrod tubes along with every sealant known to man. Some did help the leak but sooner or later it would always come back.
A few other things I have tried included cutting a groove in the barrel and installing a copper o-ring around the bore. This really helped with compression sealing -but not the oil leak.
I also grooved the head and installed viton o-rings around the pushrod holes. This cured the problem for 5 or 6000 miles but by then the o-ring would be petrified from the heat and the leak would return.
One thing I did notice on the long multiple day rides across the country was if I retorqued the head first thing in the morning I could ride all day with zero leaks. The second or third 5 or 600 mile day, the leaks would return.
So how to keep the head bolts perfectly torqued.
I tried torquing a head to an iron barrel without a head gasket and then putting it in the oven at 450 degrees for a few hours and then letting it cool. The head bolts would always loose torque.
Why? Obviously the head gasket was not compressing, since there was none.
Measuring the bolts showed they had not been stretched beyond yield.
What was obvious was that the bolt holes in the head had gotten smaller and pinched the bolt showing that the tensile strength of the bolt was higher than the compressive strength of the aluminum under the washer.
So then came .100" thick hardened ARP washers. That helped but the head still lost height around the bolt holes and the holes kept getting smaller.
Then I tried the insert type washers from ARP. They kept the holes from getting smaller but the head still lost height around the bolt holes and the head bolts still lost torque.
Then I tried special high strength bolts and immediately noticed the problem became worse.
Wrong direction...
A little over a year ago I tried something new. I put the head bolts in a mill and cut the diameter of the shank down to just under the base diameter of the threads.
I reduced the shank diameter by .062" from just above the threads to just under the head of the bolt. I used a 3/16th radius on the inner corners to eliminate stress risers. Slowly cutting the bolt in the rotating 4th axis using the side of an end mill allowed me to cut mainly in the length-wise direction.
This has turned out to be the answer. The additional stretch available from the undercut bolts has stopped the bolts from coming loose and I have had no leakage now for the past 12,000 miles -without a retorque.
So what I am using now is.
1. a copper o-ring in a groove around the chamber. It really helps prevent combustion leaks into the pushrod tubes.
2. A thin coating of liquid Teflon pipe thread sealant around the push rod tubes and oil return hole. It stays soft and flexible even at 500 degrees where all the other sealants turned to charred powder.
3. .080 thick X 3/4 OD. stainless Belville spring washers under the two front 5/16 nuts.
4. Five undercut head bolts with .100 X 3/4 inch hardened stainless washers.
I am now calling this problem cured. Jim
Thanks for posting this thread. Gotta love this forum, right guys!
