6-71 Liner/Piston Compatibility

[ric243]

Good Morning,

I am finishing up a rebuild of an old Gray Marine engine, essentially an inline 6-71, two valve head, normally aspirated with older style pistons, see below picture.

The original engine had old style injectors, advanced timing, and .81 inch liner ports. On advise, I installed N65 Brown Tag injectors and new liners, but at 1.05inch and set standard timing. I kept the old pistons and just replaced the rings. After the initial rebuild I could not get it to start, it seemed to be down on compression and I am wondering if there is incompatibility with the larger liner ports and old style pistons. I see that my pistons have all 4 compression rings grouped together, whilst a lot of the cylinder kits being sold have the top fire control ring a lot higher up.

With the change in liner ports should I have replaced the pistons also?

Another thing is that the service manual I have states that all 2 valve heads should be run with standard timing, but elsewhere it states that the N65 brown tag should be run with the advanced timing. What should I do here? I'm not sure of the difference between white and brown tag is.

I also cannot find the differences between a 71, 71E, 71M or 71N. Anyone know? I understand that the 71T is the turbo.

Any help appreciated,

Richard

 

[ric243]

Looking at the cylinder kit options available it looks like it's either a 1.05" liner with 18.7:1 CR pistons and a 4 valve head, or a 0.81" liner, 17.5:1 CR pistons with 2 valve head.

Can I run a compression ratio of 18.7:1 with a 2 valve head!?

 

[Lepke]

Yes you can run 18.7 CR. You should have bought a used 4 valve head. Also Speed of Air pistons (dimpled) give a 20+% better mileage.
I have a pair of 1947 671s I put 4 valve heads on and got a 1+ kmpg increase. More power, too, if I wanted it. 71m and n are later Detroit models that may have come with 4 valve heads and more hp.
The ports should work with the pistons. Once the piston is above the ports, the cylinder is sealed. Make sure the sleeve height to the block is within range. There are shims than go on the sleeve bottoms to set the height. You need an old style head gasket, it seals better. Also make sure the Ring end gap is correct.
My guess is if you got the sleeves and head gasket right, with new injectors, you got the rack or governor wrong. Do you know that fuel is being returned to the tank? Maybe you didn't crank long enough to get fuel. I use a 12v pump on a new engine to make sure I have fuel thru out the system.
When a Detroit doesn't start, heat helps. I have block heaters. Also a fully charged battery or a start booster will turn the engine over faster. A diesel needs heat, compression, and fuel. Normally above 35° the compressing of the air heats enough in a couple revolutions to start. If you have a water muffler, drain it.