Compression test

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Player#3

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Hi,

For a 75 NC 850 Mk 3 - what would a good to average reading for pressure be for a wet/dry compression test.

is there any point in just changing the piston rings or does it always mean a rebore and new pistons & rings ? Mileage is low but it has been sat for many years unused.

Regards,
Sean
 
Player#3 said:
is there any point in just changing the piston rings or does it always mean a rebore and new pistons & rings ? Mileage is low but it has been sat for many years unused.
Click to expand...

How can we say, until you have done the compression testing.

Best results are from an engine that is still hot, and has just been run.
If its not running yet though, this could be difficult...

Its not uncommon for long stroke engines to wear the rings, with not a lot of bore wear.
So just a re-ring may be enough.
Hard to say without having the head off, and see how the bore measures up and the pistons look.
If the compression is good and the engine is still in good condition, an oil change and new plugs air filters etc may be good to go.
Maybe.

If there is a rust mark in the bore where the pistons have been stopped, it may need a rebore and new pistons etc.
Depends on your luck, and where/how its been stored....
 
How was it stored? Dry? Garage? Shed? Have access to a borescope? Be sure to hold the throttle WFO when doing the compression test. Most engines have at least SOME rust stains/damage after sitting that long. My engine ran well, no smoke, only 80 lbs. compression. Power was down. Get back to us, let us know.
 
Player#3 said:
Hi,

For a 75 NC 850 Mk 3 - what would a good to average reading for pressure be for a wet/dry compression test.

is there any point in just changing the piston rings or does it always mean a rebore and new pistons & rings ? Mileage is low but it has been sat for many years unused.

Regards,
Sean
Click to expand...

A light hone and fresh rings is a common rehab if the cylinders aren't scored. I normally get 130-140 psi from my 850 in a compression test, depending on how hard I kick. Below 120 would concern me.
 
A compression test is relative. What they really determine is equality or inequality between the cylinders. Even if below 120, if they are equal they are ok.
 
Plain compression test worth while and easy to do, enough to get idea if ok to run some more or dig into rings or valve sealing, though in practice more efficient to just pop in new rings and see what happens and use caution to do anything much to bore surface, unless obvious about time to re-bore. Leak down test are really fun with a big bore 360' twin and not air motor through what ever is restaining rotation. I always want to see a video of someone else's attempts. There is a chart somewhere maybe one the club tech books that lists expected compression valves per model of Cdo. Combats had highest even with 2S cam bleed off.
 
Player#3 said:
Hi,

For a 75 NC 850 Mk 3 - what would a good to average reading for pressure be for a wet/dry compression test.
Regards,
Sean
Click to expand...

A good stock 850 top end will show 130 - 150PSI. Nice if its on both cylinders!
 
Ok, now a full keyboard, a little more than the first response. You MUST MEASURE if you have any hopes of a successful engine repair. After the compression test, if it is determined to be low, upon dismantling the top end, the bore must be accurately measured. The workshop manual gives the wear limits.
If your time is utterly worthless, then you could slap rings in it and hope for the best. Not an approach that I could recommend.
 
Hi,

Thanks for the reponse and comments. I found some old posts after using a few other search words with similar info and figures.

I have now tested and got 90/100 psi so difference between ccylinders not too bad although lower than expected.

Someone mentioned that wet sumping would reduce the value so as the sump has a lot of oil in it, even coming out of the timing plug hole, I will drain it and try again.

Regards,
Sean
 
If it sat for many years,you're likely to have some valve leakage which will lower your readings.That valve leakage can persist,even after you've run it for weeks or months.I've even found this on engines that haven't been started for a year.

There is a fast cure that usually works.Adjust the valve clearances and remove the exhaust pipes from the head.Start the engine and give it a few rev-ups.Open the throttle hard,and fully close it just before the revs get too high.When the engine slows down,do it again.Keep doing that for about 30 seconds.
When the engine cools down,check the valve clearances again.The next compression test should be much better than before.
 
>>>>>>>
In the absence of manufacturers quoted figures 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.

+ test all pots under the same conditions. Engine should be at normal running temperature; all spark plugs removed and twistgrip fully open.
Loosen the plugs and retighten, run the engine then remove the plugs. This removes loose carbon from the thread which could lodge under a
valve and give a misleading figure.
+ Fit compression tester to pot #1 , kick over while watching guage. The reading will increase in steps up to a maximum value. Note the reading and how many cycles are reqd to achieve the highest reading. A low initial reading with a long wait to get up to highest reading indicates a high rate of cylinder leakage. Similarly check other pots.
+ Variation of pressure between cylinders should be no more than 10%; i.e. a high cylinder reaing of pot A of 150lbs means the other pots should be no less than 135lbs. *This is just as important as the maximum
pressure reading
+ Low compression is due to: worn piston rings/bore; burnt or badly seated inlet or exhaust valves; leaking or badly adjusted decompressor; valve clearances too tight; blown cylinder head gasket. On multis lack of compression on two adjacent pots indicates a leaking head gasket between teh two.
+ Now do a "wet" test. 3 or 4 squirts of oil in the plug hole and repeat the test. If there is a substantially improved reading the leakage
is mainly past the rings - worn or broken rings or worn bore. If badly scoered the oil won't stem the leak and the reading won't improve. If there is
little change in the reading then the problem lies with the head or valves.

Hope this helps,
Graeme
 
Player#3 said:
Hi,

Thanks for the reponse and comments. I found some old posts after using a few other search words with similar info and figures.

I have now tested and got 90/100 psi so difference between ccylinders not too bad although lower than expected.

Someone mentioned that wet sumping would reduce the value so as the sump has a lot of oil in it, even coming out of the timing plug hole, I will drain it and try again.

Regards,
Sean
Click to expand...

You must hold the throttle wide open.
 
X-file said:
I
There is a fast cure that usually works.Adjust the valve clearances and remove the exhaust pipes from the head.Start the engine and give it a few rev-ups.Open the throttle hard,and fully close it just before the revs get too high.When the engine slows down,do it again.Keep doing that for about 30 seconds.
Click to expand...

Start the engine with exhaust pipes off????
 
BrianK said:
X-file said:
I
There is a fast cure that usually works.Adjust the valve clearances and remove the exhaust pipes from the head.Start the engine and give it a few rev-ups.Open the throttle hard,and fully close it just before the revs get too high.When the engine slows down,do it again.Keep doing that for about 30 seconds.
Click to expand...

Start the engine with exhaust pipes off????
Click to expand...

Please don't do this! This will warp the exhaust valves, guaranteeing a tear-down. Nothing personal, X-file.
 
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