Detroit diesels - how are the cylinders numbered ?

This on 1993 vintage Detroit 12V71TA's.

Just curious as a recent engine survey reported condition "good but some scuffing" on a few cylinders and I'd like to go back with my own scope and see this with my own eyes....and it would be helpful to know if cylinder no. 1 is the one closest the transmission or the one most forward (since then cylinders 2,3,4 and 5 would then be obvious what is what)

[cylinders 7-12 not checked as the inspection covers are too hard to get to without removing many components in the way]

Here you go, this should help...

From the Manual

Yea, 1L, 1R, 2L, 2R, etc for right bank and left bank. Number one furthest from flywheel. Contact the engine surveyor for which numbering he used. Will likely be obvious on the boat due to one side accessible, the other less so.

Thanks guys, I suspected that order but wanted to be absolutely sure. As to calling the engine surveyor, I left a message but he never called back. I can envision a situation where they might think "what a jerk, he doesn't trust our scope evaluations ?"

Well, I figure 90 percent chance they got it right, but that nagging 10 percent of doubt makes me want to see this with my own eyes as I find it so hard to believe considering the engines only have 310 hours since total rebuilds, regular oil and filter changes and mostly run at trawler speeds the whole time, no smoke (even at start up), perfect oil pressures, perfect W.O.T. figures....the whole bit. Just hard to swallow there could be any imperfections with these engines considering how perfectly they start and run.

Plus I'm curious which exact scope they used. I plan to buy one where the camera head can articulate 180 degrees (i.e. can look backwards at the cylinder wall and everywhere else in there) and connected via USB to a huge (huge in comparison to the typical fiber optic scope monitor anyway) iMac retina quality monitor.

'course the downside of that is the darn things may look even worse to me as I will see more than they did ! LOL... still, what glory it will be if I find out I've been hookwinked....

Dune

Are you familiar with what they did for cylinder work on the rebuild? When and by whom was irebuild done? In frame or removal?

Dune said:

and mostly run at trawler speeds the whole time,

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The low engine temp from running at trawler speeds can cause soot to build up in the combustion chamber, the low loads on the engine means the piston rings are not forced out against the cylinder walls and this combination can cause scuffing of the cylinder walls... It would probably be minor but worth bearing in mind...

I've looked at a LOT of 71 and 92 liners through the air ports. Almost all off them have something that looks like scuffing. Most are not actual damage. If you pull the head off you cannot even feel them. I hate scoping cylinders, creates more questions than answers.

I a cylinder is suspect, do a compression check on that hole. Or any suspect holes.

Ski in NC said:

I've looked at a LOT of 71 and 92 liners through the air ports. Almost all off them have something that looks like scuffing. Most are not actual damage. If you pull the head off you cannot even feel them. I hate scoping cylinders, creates more questions than answers.

I a cylinder is suspect, do a compression check on that hole. Or any suspect holes.

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The "diesel guy" at the yard (near Charleston, SC) was impressed that the engine with the supposed "scuffing" had zero "blow by". But now that I know this guy is not quite the diesel guru I thought he was I don't know if that is relevant or not. How would he have checked the "blow by" in the first place....compression check ?

sunchaser said:

Dune

Are you familiar with what they did for cylinder work on the rebuild? When and by whom was irebuild done? In frame or removal?

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Yes, I have all rebuild invoice paperwork on both engines. And I checked the reputation of the rebuilders (stellar, Detroit MTU trained, etc) from their competitors (i.e. the Cummins specialists and local yards in Naples, FL) Previous owner spent over $100,000 on block up rebuilds of both engines....done about 18 months apart in time periods. Presume in frame, not removal....the engine room has over 6 foot headroom.

Dune said:

The "diesel guy" at the yard (near Charleston, SC) was impressed that the engine with the supposed "scuffing" had zero "blow by". But now that I know this guy is not quite the diesel guru I thought he was I don't know if that is relevant or not. How would he have checked the "blow by" in the first place....compression check ?

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As the former owner of a boat with twin 790hp J&T 8v92's that I rebuilt twice, I can tell you that no blow-by is a very good, and relatively rare, thing in high HP Detroits. I also agree with the comment that lack of load can cause problems with the oil rings, which can cause scuffing. FWIW, I came to the conclusion that running at a little under 90% of rated RPM (2350 --> 2100 on my engines), while keeping a very careful eye on EGT (and coolant temp) was the best way to assure long engine life. But, even then, I didn't expect to get more than 3,500 hours between majors. Fortunately, I only had 16 holes to deal with -- you have 24!

Dune said:

The "diesel guy" at the yard (near Charleston, SC) was impressed that the engine with the supposed "scuffing" had zero "blow by". But now that I know this guy is not quite the diesel guru I thought he was I don't know if that is relevant or not. How would he have checked the "blow by" in the first place....compression check ?

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Rings can be shot on a DD two stroke, any leakage from them does NOT go into the crankcase as blowby, it goes into the airbox where it will not be detected. He has them confused with four strokes. Different animals. Poorly sealing rings on a four stroke does create blowby. Sounds like he does not know what he is doing.

Ski in NC said:

Rings can be shot on a DD two stroke, any leakage from them does NOT go into the crankcase as blowby, it goes into the airbox where it will not be detected. He has them confused with four strokes. Different animals. Poorly sealing rings on a four stroke does create blowby. Sounds like he does not know what he is doing.

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You are correct. I realize now that I was confused too. I must have been recalling all the times that 4 stroke engines gave me blow by trouble. But the two-stroke Detroits have a pressurized crankcase (as I recall), and that is what makes them leak like sieves (I believe). Still, I do recall that I rebuilt once because the rings were too loose. But I see that you say the blow by won't be detected in the airbox. Couldn't it make the engine run hot and smoke alot? Is there no problem (besides reduced HP) from loose rings?

What you are up against is the fact that if you pay someone to look at an engine and give you a report of its health, they will ALWAYS find something wrong with it... If they say, "no, it's fine, it will last you for years" and it blows up or dies the next day they then look bad and get a bad reputation. If they find something wrong, no matter how small, it covers them from any come back...

Personally I would leave them well alone, monitor temps, change the oil when needed and run them at WOT every now and again, sit up top, sip cocktails and enjoy life, it's too short to worry about impending engine problems that probably don't exist...

With most DD the correct oil MUST be used .

The can will be marked 40WT CF II .

If the oil consumption is low , I wouldn't worry about what you see in the cylinders looking from the air box.

Remember if the boat will not be used for a few months , spraying oil in the air box for each cylinder will help keep the cylinder walls from rusting.

Ski in NC said:

Sounds like he does not know what he is doing.

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He doesn't.....and glad to hear further confirmation of that to be honest.

To those saying "ah, don't worry about it" ... I'm not....but guess who is ? Potential buyers of the boat....drives me crazy...

MYTraveler said:

You are correct. I realize now that I was confused too. I must have been recalling all the times that 4 stroke engines gave me blow by trouble. But the two-stroke Detroits have a pressurized crankcase (as I recall), and that is what makes them leak like sieves (I believe). Still, I do recall that I rebuilt once because the rings were too loose. But I see that you say the blow by won't be detected in the airbox. Couldn't it make the engine run hot and smoke alot? Is there no problem (besides reduced HP) from loose rings?

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DD crankcase not pressurized. It is vented the same ways as any other engine.

If rings are not sealing well, first hint is heavy smoke on cold startup which persists as engine warms. A big puff is ok, should be clean in 15sec.

Ski in NC said:

DD crankcase not pressurized. It is vented the same ways as any other engine.

If rings are not sealing well, first hint is heavy smoke on cold startup which persists as engine warms. A big puff is ok, should be clean in 15sec.

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In weather 60 degrees and warmer mine don't smoke even at startup....and they start the instant the start button (or toggle, if at the helm) is pushed. Oil pressure 30 to 40 psi at cold idle.

The age , compression on a 2 stroke DD is usually measured by the coolant temperature that the white smoke (incomplete combustion) goes away.

Ckear exhaust ,Under 120F , the engine is close to new

140F the engine is in the mid life area thousands of hours still left.

160 F the oil consumption may be a an expense, the time for an inframe is coming., but the engine is still reliable , if you can afford the lube oil .

Ski in NC said:

Rings can be shot on a DD two stroke, any leakage from them does NOT go into the crankcase as blowby, it goes into the airbox where it will not be detected. He has them confused with four strokes. Different animals. Poorly sealing rings on a four stroke does create blowby. Sounds like he does not know what he is doing.

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I was inclined to agree with you but just glancing thru the Detroit 71TA shop manual in the troubleshooting section, I ran across the below concerning blow by. Thoughts ?

For high crankcase flow to be due to piston rings, it would have to be failure of the oil control rings at the bottom of the piston, which I have never seen fail. The compression rings at the top of the piston separate the compression from the airbox. Those do fail and the air flow leaking by then goes into the airbox, not the crankcase.

If you want to argue this point, spend some time getting familiar with the layout of the two stroke and how all the parts work together.

Ski in NC said:

For high crankcase flow to be due to piston rings, it would have to be failure of the oil control rings at the bottom of the piston, which I have never seen fail. The compression rings at the top of the piston separate the compression from the airbox. Those do fail and the air flow leaking by then goes into the airbox, not the crankcase.

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OK, so bottom line is someone supposedly educated in Detroit 2 strokes declairing they are impressed there is no blow by in my Detroit 2 stroke diesel, is pretty much meaningless, yes ?

It's not meaningless for there to be low base pressure/flow, in fact I check that myself. But base pressure (aka blowby) does not indicate the health of the compression rings and liner, which is the weak spot on these.