Leak Down

[IceBlue]

Just can't find any info on this topic on small blocks.

What is a good or great value (%) on the down leak on an SB.
I have a V35C which holds a 10% leak down. Good? Bad?
I think it's good, since the PO dropped a very worn cylinder on the bike which gave a 20% value, proberbly due to overheating the cylinder caused by way to early ignition. Replacing it with a low mileage cylinder brought down the down leak to 10%.

The bikes has only done 17.000km and the good side also gave a 10% leak down.

Cheers
Brian

[pressureangle]

10% is a pretty good number on any engine, if it's consistent among all the cylinders. There are many considerations and verifications you can try;
1. Be certain that you rotate the engine so that the piston is approaching TDC when you test; this ensures that the rings are seated on the piston lands. This is huge.
2. Spray WD40 or light oil in the cylinders, just enough to ensure the cylinder walls and rings are clean and wet. This can account for as much as 5%. It also gives you some idea how well the pair mates; if the cylinder is out-of-round, the oil will give little improvement.
3. Be certain that your camshaft is on TDC compression. More than once I've inattentively started the process on overlap. :/

Test in 2 or 3 places before TDC to confirm findings, you shouldn't see significant change from 90* to 10* TDC.

With care on newly assembled parts, you can see as little as 1-3% leak. After a few hundred miles, it's hard to see less than 5%.

On most engines, the majority of leak is through the exhaust valve seat. If the pipes are off, spray a little WD40 in around the exhaust valve to see how much air is coming past- one would hope it's not enough to hear plainly.

[moto-uno]

  If when doing this leak down test , don't be afraid to put your ear close to the end of the muffler and also at
the intake  at the carb . It's been a while since doing one of these and I'm not too sure if 10 % is all that good .
Any sound at either of those places might suggest valves seats ( or valve clearances ) .  Peter

[rschrum]

For the age and miles, 10% is fine. Get that ignition timing dialed in and ride it.

[IceBlue]

Thanks guys! Basically the answer I was hoping for. This little engine purres like a kitten. Ignition all fixed. Valve leaks checked. There are no shoulders in the liners, the rings are within tolerances. This is a cute little Guzzi. Rather dead up to 4000rpm, then she wakes up 

Only one persistent issue. She spills oil out of the breather box's drain when pushed hard, ever if she is not overfilled - oil level is equally between oil level markers.
Checked the heads oil drain, and they are ok, however, she has upgraded oil dowels from 1mm til 3mm. That means a lot more oil goes to the heads.

I'm thinking about downs sizing to 2mm to limit the amount of oil going to the rockers.

What are your thinking?

Cheers
Brian

[jacksonracingcomau]

Thanks guys! Basically the answer I was hoping for. This little engine purres like a kitten. Ignition all fixed. Valve leaks checked. There are no shoulders in the liners, the rings are within tolerances. This is a cute little Guzzi. Rather dead up to 4000rpm, then she wakes up 

Only one persistent issue. She spills oil out of the breather box's drain when pushed hard, ever if she is not overfilled - oil level is equally between oil level markers.
Checked the heads oil drain, and they are ok, however, she has upgraded oil dowels from 1mm til 3mm. That means a lot more oil goes to the heads.

I'm thinking about downs sizing to 2mm to limit the amount of oil going to the rockers.

What are your thinking?

Cheers
Brian

Apparently one of expert fixes for Lario that didn’t make any sense to me, rockers well lubed with std feed, fixing a problem we don’t have and creating your symptom exactly what you’d expect.
Put original restricters back in and report back, I’m interested.
My Lario revs to 9 all the time, no breathing issues whatsoever, rockers well lubed, std restricters
But 10% is on my limit of acceptable, set of rings would be money well spent IMHO, checking valve seating a matter of course along with springs etc

[IceBlue]

Apparently one of expert fixes for Lario that didn’t make any sense to me, rockers well lubed with std feed, fixing a problem we don’t have and creating your symptom exactly what you’d expect.
Put original restricters back in and report back, I’m interested.
My Lario revs to 9 all the time, no breathing issues whatsoever, rockers well lubed, std restricters
But 10% is on my limit of acceptable, set of rings would be money well spent IMHO, checking valve seating a matter of course along with springs etc

The bike in question is a V35C - but the principle is the same.
My two Lario's have 2mm restrictors - stock confuguration. Guzziologi quotes Guzzi recommending going from 2 to 3mm. I have not upgraded. A German Guzzi expert (Martin Hagemann) recommends not to do it.
I'm suspicious of the 3mm restrictors (dowels) being the problem on the V35C, as the stock config. is 1mm. It revs up to 9.000rpm, and that will send oils out the breather system.

Ciao

[Cdn850T5NT]

Re the leakdown tester, do you mind my asking what orifice diameter and length you are using (if you know) and/or what brand of tester and are you using... and 80 psi or 100 psi as supply pressure?. If it was a piston-engined aircraft of less than 5" bore size, it'd be 80 psi, 0.040" dia x 0.250" length orifice ("tube") with 60° "approach" and "exit". FAA is very specific on this.

The reason I ask is that 10% leakdown depends on all of these factors... and while it's standardized in the aircraft business it does not appear to me to be standardized with automobiles or motorcycles.

[mondtster]

Re the leakdown tester, do you mind my asking what orifice diameter and length you are using (if you know) and/or what brand of tester and are you using... and 80 psi or 100 psi as supply pressure?. If it was a piston-engined aircraft of less than 5" bore size, it'd be 80 psi, 0.040" dia x 0.250" length orifice ("tube") with 60° "approach" and "exit". FAA is very specific on this.

The reason I ask is that 10% leakdown depends on all of these factors... and while it's standardized in the aircraft business it does not appear to me to be standardized with automobiles or motorcycles.

All this is true, and if you want to produce some really good numbers to help sell an airplane use the larger orifice size. When buying use the smaller one. The numbers produced on the same engine will be different. (For the record, I wouldn't do this but I'm sure some guys do or have.)

Cylinder leakage testing is very subjective and while it isn't meaningless, it should be approached as simply one tool of many to determine engine health. There would likely be other symptoms of poor ring/cylinder condition before it would manifest itself by pushing oil out the breather, if it ever did that.

[Chuck in Indiana]

10% is getting to the place that I start looking for issues. It's not terrible, but it *might* be telling you something if you are attentive. Listen at that exhaust, breather, and intake. You'll be able to tell what is leaking and if you should be worried..
A "good" engine will run 5% or a little over.

[IceBlue]

Thanks guys!

This is the instructions I use.
The inner diameter of the adaptor is 6mm - sorry we left the imperial system in 1907, so I would not know what that is in parts of an inch 









This indicate that 20% is OK - which was hard for me to believe.
This gauge seems meant for car engines, so perhaps that has to go into the equation.

Ciao

[jacksonracingcomau]

Flyboys will chime in on the buying and selling engines bit
But for racebikes is good check , after a while you know when to refresh
For road bikes, generally it’s looking for fault, invaluable tool, I know my expectation with my Snap On leakdown and Guzzi road bikes, YMMV

For you, best to check other bikes you think are good, draw your own conclusions when you have benchmark.

But I want to know if smaller restricters fixed the issue

[Chuck in Indiana]

The inner diameter of the adaptor is 6mm

That isn't the inner diameter.. the inner diameter is an orifice between your 6mm ids. From memory, .045" about 1.1mm.

[IceBlue]

Thanks guys!!

[lucky phil]

Thanks guys!

This is the instructions I use.
The inner diameter of the adaptor is 6mm - sorry we left the imperial system in 1907, so I would not know what that is in parts of an inch 









This indicate that 20% is OK - which was hard for me to believe.
This gauge seems meant for car engines, so perhaps that has to go into the equation.



Ciao

You mean just like a Snap on one?

Ciao

[PeteS]

If you have to pull the cylinder off to replace something or have to get it bored, I would recommend going to Total Seal rings. I would typically get 1-3% leakdown that would last for at least 5000 miles, maybe more. You can feel the extra grunt between 1-3% and 10%. If they don't typically stock what you need they will make them up for at no extra charge. I got them for my Norton PowerMax pistons as well and my LeMans and BMW airhead. Make sure to tell them if your cylinders are Nikasil or iron liners. They use different materials for both.

Pete

[lucky phil]

If you have to pull the cylinder off to replace something or have to get it bored, I would recommend going to Total Seal rings. I would typically get 1-3% leakdown that would last for at least 5000 miles, maybe more. You can feel the extra grunt between 1-3% and 10%. If they don't typically stock what you need they will make them up for at no extra charge. I got them for my Norton PowerMax pistons as well and my LeMans and BMW airhead. Make sure to tell them if your cylinders are Nikasil or iron liners. They use different materials for both.

Pete

Nice to know. Thanks

Ciao