commando compression (2012)

maylar · Aug 8, 2012

hobot said:
I really truly await a saga report on attempts to perform useful leak down test on an intact Commando.

In practice street machine low compression d/t leaks are a non issue until smoke out the pipes or the case breather till bad enough not to start or run.

I have done leakdown tests on Commandos and yes the results are meaningful. Hissing through the intake after a valve job confirmed the job was done poorly. Hissing through the crankcase breather confirmed rings were worn and the reason for smoke under load. Both cases the leak pointer said "not so good". A tight healthy engine reads as it should, though it's only the top end that's being checked. Low compression psi and good leakdown can mean a worn cam lobe. Gotta know how to use the tools.

comnoz · Aug 8, 2012

I have done leakdown tests on Commandos and yes the results are meaningful. Hissing through the intake after a valve job confirmed the job was done poorly. Hissing through the crankcase breather confirmed rings were worn and the reason for smoke under load. Both cases the leak pointer said "not so good". A tight healthy engine reads as it should, though it's only the top end that's being checked. Low compression psi and good leakdown can mean a worn cam lobe. Gotta know how to use the tools.[/quote]

+1

Hobot · Aug 8, 2012

I am very used to people assuming they understand what I write but ignoring that, pray tell how you controlled the engine turning to not open a valve a crack to get worthless data on air sounds. Also pray tell what air escape out rocker box means, like me and Wes got with engine balanced on TDC nil force on drive train? Most useful info we got was that the TDC alternator mark was ~one degree advanced, which accounts for points time light showing best Combat performance at 29' instead of 28' indicated. I fumble degree wheels and time light too, so found best timing by trial error then looked to see where it landed. Leak down test in my Trixie case didn't reveal the huge ring gap problem by breather sound, only tear down and inspection did. I'm just a pilot not a mechanic. Have leak down tester, will send away cheap.

Oh yeah for extra credit explain how you controlled engine torque to rock off TDC to get rings onto possible bore leak zone if not found at TDC reversal zone?

comnoz · Aug 8, 2012

I use a breaker bar on the end of the crankshaft. Position the crank at 10 or 15 degrees BTDC and rest the bar on top of the footpeg . Then I apply the air. It will definitely show increased leakage with ring gap growth but it is best if you have a "good" reading to compare.

Air from the valve covers means air is getting into the crankcase. That means rings or a head gasket blown into a pushrod tube passage.

The last time I checked my 12000 mile original MK3 I got around 10% leakage on a snap on gauge. I used the locating plug in the MK3 crankcase to hold the crank at TDC when I checked it.

I also have a Sun gauge and the readings are nearly identical to the Snap on unit.

Leakdown past the rings does not always go hand in hand with oil consumption but if you see leakage over 15% on a warm engine then you will be losing performance.

A valve that is not sealing perfectly shows up big time on a leakdown tester. [and in performance] It's also easy to feel the air through the exhaust or intake.

I would agree that an engine with an obvious compression problem is not worth doing a leakdown test. Just tear it apart and see why. You probably wouldn't even need a compression tester to see a big compression problem, just use the kickstarter with you hand and compare the two sides. Jim

Dances with Shrapnel · Aug 8, 2012

Jim,

I think you are pounding it into one of hobots ears but as usual, it is dribbling out his other ear......

Never thought of a leak down myself - great idea.

Hobot · Aug 8, 2012

Dear dances I'm a pilot not a mechanic but consider that this Charlie Brown got the engine balanced right on TDC, so nil drive train blocking needed. No breaker bar or open primary needed. That state is supposed to be good enough to hear where the leaks are but in Trixie case that only gave sound out the intake rocker box, which we had open to make sure which piston was sealed. Please explain that with your expertise on this subject. Wes and I had a Brit Iron Blank Starting State on that one and remains another magical Commando mystery. But hearing no leaks anywhere else the leak down test was a failure to ID the reason for smoking. The hard part was taking the test further by creeping off TDC, in either direction and we did with an expedient set up, but still did not reveal air leaks sound. There is only a few degrees of Combat crank off TDC before normal valve over lap straining us to restrain it, til poof the test went caput. My frustration was it didn't help a bit, only tear down and inspection did. I've had brains about leaking out ears, more than once, so magical mystery I'm functional to ride and banter on Commandos with ya. Oh yeah the wastage range was ~20% so in that regard it agreed with what we already knew, too smoky and misfiring so time to dig in.

Hobot · Aug 8, 2012

Air from the valve covers means air is getting into the crankcase. That means rings or a head gasket blown into a pushrod tube passage.

Alrighty that solves my gap in logic, with valve cover off that'd be easier than out the oil tank. All the video I saw was for engines they left those sealed. So if your wisdom hadn't leaked out , the test would have agreed with the found ring gap over sight.

I use a breaker bar on the end of the crankshaft. Position the crank at 10 or 15 degrees BTDC and rest the bar on top of the footpeg . Then I apply the air. It will definitely show increased leakage with ring gap growth but it is best if you have a "good" reading to compare.

I use small Al plates to lock up the works where and direction. Now Dances has set me straight I will keep my tester because I have to know everything about Peel for her to get me off good.

I would agree that an engine with an obvious compression problem is not worth doing a leakdown test. Just tear it apart and see why. You probably wouldn't even need a compression tester to see a big compression problem, just use the kickstarter with you hand and compare the two sides. Jim

Now that's the down and dirty seasoned Nortoneering wisdom I can handle.

maylar · Aug 9, 2012

Holding the engine stationary during the test is indeed a challenge with a Commando. The range of crank movement where both valves are closed is rather small. Best way is as Jim says, with a breaker bar on the crank nut. But that requires primary cover off. I recently did a leakdown test on a Combat motor and managed to hold things in place by slowly turning the rear wheel whilst in 3rd gear and jamming the rear brake. Gotta take all the slack out of the chains.

alexhobo · Aug 9, 2012

jsouthard said:
I just bought one of those fancy compression testers from harborb freight. I tested with the bikes cold but found 70 psi on the left cylinder and 50 psi on the right. Throttle wide open.

Why am I seeing such a difference from the 120-150 range others have post?

im glad im not the only person to have this problem

iceteanolemon · Aug 10, 2012

alexhobo said:
jsouthard said:

I just bought one of those fancy compression testers from harborb freight. I tested with the bikes cold but found 70 psi on the left cylinder and 50 psi on the right. Throttle wide open.

Why am I seeing such a difference from the 120-150 range others have post?

Click to expand...

im glad im not the only person to have this problem

yup me too. i was testing cold though and not with throttle wide open etc. I have a good tester too, at least its been used many times on cars without issue. I think its jsut the approach, gotta do it like the guys say. I still need to do mine, this weekend!

iceteanolemon · Aug 10, 2012

OK I got the bike warmed up and test rode it around the block a few times. Its runningwell considering the blistering heat outside...

I didnt get it out and about too much but I figure I got it hot enough to test compression...

Heres the right side.

And the left side..

I got the bike nice and warm then pulled both plugs. They were hot. Then I opened the throttle fully while kicking it over. Thats my readings.

So are these normal-ish? I am pretty surprised at how even they are. Hopefully they represent a good amount of compression.

Bernhard · Aug 11, 2012

They are both within acceptable parameters.
nothing to worry your little head about really

maylar · Aug 11, 2012

Looks excellent to me

pete.v · Aug 11, 2012

I am not going to say they are in high range because pressure can be relative to the gauge and the tester.
The Key is that they are very even. If they were 100psi but even, it would be ok.

iceteanolemon · Aug 11, 2012

Thanks for the confirmation. I too take the actual pressue with the grain of salt. I am really just confirming both cyl are in ballpark with each other and I am happy with the readings.

Now if I can look for some leakdown testing stuff later that may be fun too...

Hobot · Aug 11, 2012

Just to keep the mental oil pressure flowing over a weekend, with most away from work stations... Btw Norton did issue some Commandos with oil pressure gauges, I've never seen one, has anyone?

http://www.nortonownersclub.org/support ... l-pressure

http://home.clara.net/captain.norton/cnn4sec23.html

Rohan · Aug 11, 2012

170 psi on both cylinders is what I got on a freshly rebuilt and run-in 850 Commando.
So your ~170 psi on both cylinders is spot on the same - and excellent...

iceteanolemon said:
OK I got the bike warmed up and test rode it around the block a few times. Its runningwell considering the blistering heat outside...

I didnt get it out and about too much but I figure I got it hot enough to test compression...

Heres the right side.

And the left side..

I got the bike nice and warm then pulled both plugs. They were hot. Then I opened the throttle fully while kicking it over. Thats my readings.

So are these normal-ish? I am pretty surprised at how even they are. Hopefully they represent a good amount of compression.

Rohan · Aug 11, 2012

Bernhard said:
They are both within acceptable parameters.
nothing to worry your little head about really

Within "acceptable parameters". What does this statement mean ?

170 psi, give or take a few psi, is what a brand new run-in 850 engine will give.
It can only go down from there.....

Hobot · Aug 11, 2012

Extra good each side is so close too as differences in combustion spikes can give some horizontal imbalance that might be felt. I've had that happen when carb bowl or gasket fell away, but was pretty subtle if happens on the fly. So what cam and compression ratio, gasket, base plate, to judge the sealing goodness? I've heard Combat can be kicked into the 180's. Yours seems rather good by below.

expect the following pressure from different Compression Ratios:
6:1 88-118;
7:1 103-147;
8:1 125-162;
9:1 140-176;
10:1 162-191;
11:0 176-206.

Rohan · Aug 11, 2012

Where do you get those numbers from Steve ?

commando compression (2012)

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