I installed S&S 110 kit on my TC breakout ~2 years ago. Just out of curiosity (bike is running fine), I had a leak down test done recently, and it indicated 6% front cylinder and 9% in the rear jug. Both are within “acceptable” ranges, but I can’t help wanting to do something about the rear jug’s leak down.

2 questions: Is it common for the rear cylinder to have higher leak down than the front? If you had 9% ld, would leave it alone or try to improve it (perhaps replace piston rings)?

thanks in advance for any advice

 

Cylinder leak down tests are a terrible indicator of engine health, I wouldn't give it another thought.

 

If the bike is running good, I wouldn't even bat an eye at those leak down results.


Real question is, why did you feel the need to perform a leak down test?

 

If it is the cylinders (there have been issues)new rings won't do much on their own, if it's not drinking oil and there are no other symptoms then as above try to forget about Mr 9% and enjoy the riding, if something pops up down the track that needs addressing then that's when you will need to look at it.

 

Thanks guys. It’s running fine, so I’ll just forget about the 9%!

 

Dont go looking for problems that arent there........drives ya nuts!

 

Leak down is a test what you do when you want to take the motor apart.

 

The rear cylinder typically will run hotter than the front so it's possible the ring seat is not as good as the front cylinder but is still nothing to worry about.. Keep an eye on you plugs for possible oil in the future.. IN THE FUTURE..

 

Your fine. The rear wears more then the front from what I have seen but that was good readings.

However, is there a rating for a leak down in modern Harley services manuals? I always use a good old fashion compression reading on a warm engine. The throttle body needs to be blocked wide open.

Now if it's not to spec. then, I use the leak down to see if it's leaking thru the valves or by the cylinder. Only thing in my opinion there really good for.

But your reading was pretty good. Not sure why your paying for that.

Tools are available for free at AutoZone.

Paying for that is just opening yourself for someone saying you need to do something to it.

At 15% is acceptable. Even at 20% for big piston diameters is not unusual and runs fine.

I saw my 93 YJ 4 cylinder at 45% and it was still running OK but obviously down on power. It had 300,000 miles on it and the rings were gone from overheating it when a freeze plug rusted out under the exhaust manifold.. I did a in frame overhaul on it however.

It wasn't even smoking but cats hide that till they stop up.

I saw a turbo Hyundai my student had at 190,000 with 6% on all 4 cylinders. However, it has pretty small pistons.

 

Too many think that leak down is the panacea for troubleshooting.. Truth is that there are many variables that can through the readings off. Some are as simple as piston to cylinder clearance, temperature of the motor, relative ring location when test performed (all rings spin in the grooves except most 2 strokes), position of the piston in the cylinder that the test is performed.

The idea is to use a freshly built motor and take the measurements were all the variables are the same then look for changes.. Doing a leakdown test on a motor to troubleshoot issues without recorded initial measurements is only good for gross errors, say anything above 10-15%.