Minarelli 2T rings

[duosport]

oppps! you're right. I confused Circumerence with diameter.

[duosport]

So if I had a micrometer and a piston bore gauge I am to measure the bore at various locations and find the largest diameter registered. No doubt this would be near the top of the bore. Next I am to measure the piston. I make this masurement in the skirt area as the piston is tapered to compensate for expansion of the head from heat. I have made my measurements using the feeler gauges at the very top of the piston. Well the piston is tapered so the measurement is bound to be incorrect. I will make my measurements again turning the piston around and measure the skirt of the piston to the cylinder near top.

[duosport]

Now I made the measurement of bore to piston with the feeler gauges in the skirt area of the piston. I have very different results of course. With my old piston in place upside down in the bore I measure the skirt area of the piston as compared to the near top of bore where one gets most of the wear. I do not have two feeler gauges small enough to fit. I can only use one gauge on one side I used a .0015 gauge and managed to squeeze it inbetween piston and bore wall. It was fairly snug. I cannot fit a .002 at all in anyway.

So the difference between bore and piston at skirt is .0015. That is totally in tolerance.

[duosport]

I have also checked head and cylinder top for flatness. There I have some small issues. While the guide says .002 as maximum, in the Malaguti service manual it says .0012" is maximum deviation. I have both .002 deviation from flat in the head in one area and .002 deviation in one area of the top of cylinder. This is at the farthest extreme for general engine specs and out of spec by the Malaguti service manual. I think because the engine is water cooled the specs are tight.

[stepthrutuner]

So now you can check out the ring side clearance and back clearance.

[duosport]

The ring side clearance is max .005

[stepthrutuner]

Any idea what it's supposed to be for your particular piston? My book says between 0.04 and 0.1mm (0.0016" and 0.0039") for 2t pistons in general.

[duosport]

The Malaguti sepcs are .006 - 014 of an inch


I have .005

[duosport]

.005 on the replacement piston and .006 on the original piston.


All of the numbers I am coming up with show the original piston is closer to spec.

[duosport]

I have been working of the truing up the flatness of the cylinder to and the head. I wrked in a plane of glass with 800 wet paper. I used a figure 8 motion and trued up the head pretty quickly. The cylinder took much more work. The metal is denser and the profile thicker but also tyhe deviation was greater. I have them both so flat that my smallest feeler gauge (.0015) will not slide under. The specs for Malaguti are .0014 of an inch tolerance. So I have it now in tolerance.

[duosport]

I think my problem was the seal between head and cylinder. They both deviated from spec and the deviation occured in the same area when put together. So .002 in both added up to .004 when put together. This occured in the inner seal area between bore and water jacket. I think pressure was blowing from bore into the water jacket. The seal outside of the water jacket to the outside air was good. I would have seen no evidence if the pressure went into the cooling system short of oil in the reservoir. As I had drained the cooling system and put new in before the initial ride, there was not a lot of time for exhaust to contaminate the coolant. I rode it maybe 10 miles before the second disassemble.

[duosport]

So here is what I plan to do. I shall use the original piston as it is closer to spec. I am going to assemble all and give it a go. Before I do is there anything else I need to do?

[2strokd]

I think your doing great, and learning loads while you go.
I would make one of these if you can? Its a pressure tester for finding leaks in our 2T seals and gaskets, and or mating surfaces. Theres a link here some were....

[duosport]

Well if I had that pressure tester I bet it would detect a massive leak. Obviously I am loosing seal somewhere to only register 92 PSI

I assembled the scooter all correctly. I fired it up. I idled if for several minutes then took it for a ride. Again it rode exactly as before. No power. I took it back to the garage and did a compression test and again 92 PSI! Problem not solved.

I did notice something however that I had not before. Though I inspected the threads of the spark plug opening, and they appeared OK, I did however notice a lot of play in it today. So I screwed in the spark plug 1/4 way and I could rock it back and forth a bit. I screwed it in half way and could still rock it. I screwed it in 3/4 of the way and still I could rock it back and forth a bit. I think the threads are worn out. There is no evidence of cross threading so maybe it is just generally worn everywhere in the thread.

[duosport]

So is this sort of thing my viable option?

www.carparts.com/autoparts/ProductBrowse?storeId=10401&Pr=p_Product.CATENTRY_ID%3A2004268&productId=2004268&catalogId=10618&N=111%2B10718%2B600022196&rlid=cp_group_detail