1GM piston height/compression

[cindersailor]

Having reached the final stage of the top-end rebuild of my second hand Yanmar 1GM before I fit it into the boat I find that it refuses to start without providing the intake with some hot air from a paint stripper. Runs fine once started. Squirting some oil into the bore makes very little difference to starting so I assume the rings are OK. Spoke to Cellar marine who have been very helpful in the past who suggested I need to check the piston height above the top of the bore which should be 0.37mm (15 thou) and fit a new gasket. I find that my piston is just 0.15 mm above the bore at TDC. I find it hard to believe that 0.22 mm in a stroke of 72 mm will make sufficient difference to compression to prevent starting. Interestingly the specifications say that the piston to head separation at TDC should be 0.7mm. The new gasket is 1.4 mm thick (as is the old one) which indicates that it must be reversibly compressed to 1.07 mm when the head is torqued up - possible I suppose.

Does anyone have any suggestions what else could be causing the starting problems or do I need to bite the bullet and buy and fit a new con-rod to gain the extra 0.22 mm? A thinner gasket would be an attractive solution if it is a compression problem but I doubt this is available.

<hr width=100% size=1>

 

[steve28]

I think its almost certain that it is the cause of your problems, how are you measuring the gap, i used lead solder and turned the crank by hand so as to crush the solder and then measured the crushed bit.
Its so common it would seem on the gm engines, i have found Chris French at French Marine is the best for advice, been yanmar agent for 30 years tel. 01206 302133
Why did you rebuild the head ?
Any chance it took some water in the exhaust ?


ps try posting your problem on http://www.yanmarhelp.com/msgboard.mv
steve

<hr width=100% size=1>

 

[cindersailor]

I measured the piston height by putting a sraight edge across the top of the piston at TDC and measuring the gap to the bore top surface (which the gasket sits on) with feeler guages. The rebuild was a result of poor compression which turned out to be poor valve sealing which was easily solved by regrinding. This was probably due to a bit of corrosion on the valve surfaces as the engine has been in storage for 4 years. I know that it has already had a new poiston/con rod due to water in the cyclinder bending the con rod so perhaps it happened again before I bought it, but 0.22 mm seems very little.

Have posted on the Yanmar forum as you suggest, will wait to see the response.

<hr width=100% size=1>

 

[Heckler]

i dont know specifics about your engine but can talk about diesels engines in general, normally where they talk about piston hieght clearances and its importance a selection of head gaskets are offered in different thicknesses.
but i would first of all, before going down the route of chasing my tail just on assumptions, get hold of a compression tester and check that it is that that is the problem. than once i knew that it is definitely compression that is the prob then i would be at the stage where you are now. do the lead bit to check the clearance so that you get measurements with the head gasket squashed and in a running condition.
use your head as well in working out what is happening, what is the swept volume (pi r squared times h) and then put piston at top dead and pour a liquid into the combustion space and carefully measure its volume, divide this into the swept volume and you will get the compression ratio, usually about 22:1 but check in the handbook.
stu

<hr width=100% size=1>http://www.beneteau-owners-association.org.uk

 

[Birdseye]

diesels only need three things to fire - air, fuel and compression. if the first 2 are ok, then it must be the third. probably best to check first with a compression tester - first without out putting any oil in the bores and without having turned the engine over for a while, and then if necessary after putting some oil down the bores.



<hr width=100% size=1>