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Mk 1 Cylinder pressure???

kiwi croc

kiwi croc

Tuned and Synch'ed
Joined
Apr 8, 2011
Messages
53
Location
Greymouth NZ
Hi there, I have a 1977 Mk 1 with 74,000 miles on it. I had the heads done up about 8 months ago with new exhaust valves and guides replaced. motor is running good but has started to blow quite a bit of oil out the engine breather hose. I checked cylinder pressures 8 months ago and they were 170 left and 160 right, they are now 165 left and 150 right, I have done about 5000miles hard riding between checks. I will check engine breather box valve operation to try and stop oil loss out breather but wonder if I should also check rings etc? Dose anyone know what cylinder pressure i should have?? Cheers Croc. ps. couple of pics of my bikes for interest. Upload 2018 12 18 22 55 42 Upload 2018 12 18 22 59 46 Upload 2018 12 18 23 8 10 Upload 2018 12 18 23 10 49
 
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A leak down test will tell you if the rings are going. I think the test will just confirm that. I've learned that doing valves and not doing rings just means you have to go back soon for the rings. I've found that 170 to 175 psi is about what a healthy engine yields on a compression test. There will be some variation based on accuracy of gauges. Next time you want to do valve guides look at having K lines installed.
 
I am with John a firm believer in leak down testing - it tells you if you have a problem and where it is by the hissing, quickly and simply. There will always be hissing out of the crankcase but the valves should never leak. If you don't have a leak down tester an air fitting brazed to a sparkplug with the ceramic removed is a great and dirt cheap diagnostic aid. You can make a leak down tester with a regulator, gauge and a 1mm orifice between the two and a quick disconnect. Normal operating pressure is 60psi (0% leakage = 60PSI). Put the cylinder on TDC with both valves closed engine hot and set to TDC both valves closed, set tester to 0 and connect to engine. Beware that the engine can rotate quickly when under pressure.

A compression test should be always taken as a general indicator - there are too many variables: battery health, barometric pressure/humidity, throttle position, engine/oil temperature. From 170 to 160 is within

In addition to K-lines which are pretty cheap If you have a chance to move to Nikasils (Gilardonis) I would - it is the single best mode you can make. Your engine will easily pick up a few HP just by reduced blowby and they last. If you have chrome lined cylinders and I think there were some in the early lemans definitely go to nicasils - the chrome flakes off and destroys bearings. I have seen leak down as low as 2% with nicasils. With cast iron you are l
 
Update; I done a leakdown test and it was not good - 30 to 40 % leakdown!! I have since replaced the rings afer honeing and de-liping the cast iron bores. The bores and pistons are in good condition but worn, so next time it will be Gilardonis. I done another leakdown test after retoqueing the heads for the first time and i got 5% one side and 6% the other so was quite happy with that.Bike is running very well with very little oil from breather (i attached a clear catch bottle so i could check). This bike is just GREAT to ride!!
 

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If you have a lip on the bores definitely time to look for Gilardonis - the factory does them in batches and you have to snag them when and where available because you may not be able to get hold of them for some time. Last time I ordered them through Agostinis in Mandello at a decent price even with shipping. Harpers is also a good source.

Less than 10% is pretty good with iron bores.

BTW the lemans breather can definitely be improved - I get rid of the valve inside the box (stout screwdriver and hammer) and stuff it with stainless steel wool to help air-oil separation and stick a stainless EGR valve (reed valve) at the exhaust end of the box - sorry don't know make or which cars it was fitted to but it is a fairly standard 80's part. The valve is important to stop moist air entering the crankcase and causing mayonnaise - probably not a big deal if you are in California but a big deal if you live in Holland. I also put a catch can to monitor things.
 
Thanks for your reply, The lip was minimal but had the tool to fix it so we did and it worked a treat.I have given the origial valve in the breather the chop and installed a later model one way valve i think from a V11 in the engine tube. I re-toqued the heads for the last time yesterday and checked valve clearances,also changed oil ready to go for a ride tomorrow. Thanks for the advise about the Gilardonis, Cheers.
 
john zibell said:
They work on their own schedule. No one but them knows when standard bore kits would be produced.

Charlie, would these work? http://www.mgcycle.com/index.php?main_page=product_info&cPath=68_129&products_id=2129 They are round head standard size
Click to expand...

I doubt we'll ever see standard 83 mm bore Le Mans Gilardoni kits again, but you never know.

The Le Mans pistons have a higher dome than the T/T3 kits, so compression would be considerably lower if those were used.
 
I would take the stock pistons & have a set made 1st oversize the bore cast cyl to match. This is how it was designed to have done, they used to have 1st & 2nd size pistons. OR get a set of new liners from LA and reuse pistons. 1000 G5 & SP are the same way.
 
In my experience if you leave everything the same there is not much of a performance gain with the big bore i.e. the big block is limited by airflow. Some think it is a 'Sweeter' motor with the stock bores - I haven't really noticed that much of a difference with unmodified motors. It may give a bit more low end grunt.

However I have noticed that the big bore really helps with combustion if you reduce the quench zone to the minimum you can get away with - I think I left it at 25-30 thou. It greatly helps with pinging while upping the compression. I had a online discussion with Pete Roper a while ago and I think he found the pretty much same thing. Do check interference with plasticene before you start the engine!

If you reduce the quench you can knock back the advance 3-5 degrees. If you do effective porting the big bore comes into its own.

I think Charlie is right - getting stock Gilardonis is probably not going to happen. I personally wouldn't spend my money on liners - the nicasils are really so much better. Unless you are an originality fetishists - and nicasils were not original - I wouldn't give it a second thought.

Do have it rebalanced - this may account for some people's opinion that it is sweeter with the original bores.
 
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