GM 671N Marine Diesel

[drcoastline]

I am sure I will get the correct answer here faster than posting in the Diesel forum I am subscribed to for information on these engines. The knowledge here amazes me.

My question is basic and I think I already know the answer I just wish to verify before spending any money.

My boat has twin vintage 671N engines. The gauges are mechanical. A few years back the Port side tach crapped out and the marina replaced the original AC tach with a Stewart Warner tach. The SW tach works fine but looks terible as it does not match the rest of the gauge cluster. I want to replace the SW tach with an AC tach. I have found a few NOS gauges that look correct but want to verify that they will work in a marine application?

Am I correct RPM's are RPM's? As I recall the revolutions are taken from the fly wheel so the tach should work with out issue?

Can anyone confirm?

[FLYGEEZER]

Can't say for sure. But I know the 671N (4 valve) very well. Do ya got 65 brown tag injectors for 238 hp? I'm bettin a mech tach is a mech tach and will work . Love to see pics of the engines. Best of luck!

[1990C4S]

I thought tach's typically ran from the ignition. On my diesel car conversion I had to get a signal from the alternator.

[drcoastline]

Hi FLYGEEZER,

Thank you for your post. You nailed it. That's exactly what I have.

These are the best I have of the engines right now. I'll try and get some better pics this week when we go back to the boat.

[drcoastline]

Originally Posted by 1990C4S

I thought tach's typically ran from the ignition. On my diesel car conversion I had to get a signal from the alternator.

What year is your car? Your car probably has an electronic tach? This is mechanical. I believe mechanical tachs can get signals from different areas. This application I think comes off the fly wheel but even if I'm wrong I don't think that matters.

[chocolatelab]

Id like to see the boat those belong too? Very cool.

Or is that some kind of back up power?

[daepp]

Completely off topic, but an old-timer friend of mine had a late 60's/early 70's 65' Hatteras pleasure craft in Newport Beach harbor. He was a CPA who knew value when he saw it, bought it in pretty rough shape, then did much of the restoration work himself. It was a beauty!

In that application he had a central hallway (gangway?) that ran below decks, and at one point there was a small man-door on either side of hallway that accessed separate engine rooms. I was always fascinated with how organized everything was there, and clean, and everything in them was there for a purpose. Back when red, multiple drawer tool chests were a luxury, he had one in each room. I commented how awesome they were, and I'll never forget his reply: while he loved that boat, and used it often, he said he'd never keep it going without constant maintenance and ready access to ALL those tools.

Sorry for the OT reminiscence...

Good luck with your tach - those engines look legit!
And I second the request for more pics Doc - both inside and out, and of those cool gauges.

[drcoastline]

Chris- These are her standard power and original engines to the boat.

David- thanks for your post. Great story. This boat was also bought "rough". She is in the process of an extensive restoration. Work stops for the summers so our family can enjoy her. The boat is structurally and mechanically in very good condition. But it's time for a refit and update of some systems and she is in desperate need of a cosmetic update.

I don't have any photos from this year yet but here are a few from last summer.

[drcoastline]

Here's a photo of the helm with the mis-matched tachometer.

[greglepore]

Yummm. If you ever need a mate (the nautical kind) I'm in...

[Porchdog]

Did the marina scrap the original tach? I would expect to be able to get a mechanical tach repaired.

[widgeon13]

Beautiful boat!

[drcoastline]

Greg- We can always use an extra mate. There's always something to be done.

Porchdog- I don't know what happened to it, it was gone before we purchased the boat.

Thank you Widgeon13.

[wdfifteen]

I know squat about boats, but I have some experience with Detroit diesels. The ones I've worked with had governors and the tach drive came off the governors. Some I've seen come off the injector pump drive, and some off the cam. Your replacement tach is going to have to be matched with whatever drive your engine provides. If it comes off the flywheel the cable speed isn't going to be 1 to 1 with the engine speed.

[drcoastline]

Originally Posted by wdfifteen

I know squat about boats, but I have some experience with Detroit diesels. The ones I've worked with had governors and the tach drive came off the governors. Some I've seen come off the injector pump drive, and some off the cam. Your replacement tach is going to have to be matched with whatever drive your engine provides. If it comes off the flywheel the cable speed isn't going to be 1 to 1 with the engine speed.

Thanks widefifteen this is the type of info I am looking for. I could be wrong about the drive coming off the fly wheel, it may very well come off the governor, injector or cam. I assume the drive is in the original location and the replacement SW tach is working off the original drive location. That being said the actual tach should be OK? I understand there may be some calibration necessary but aside from that it should work?

Thanks again

[wdfifteen]

If I were you I'd be looking at the original AC tach to see what model number and other information it may have on it. (Like input rpm to readout ratio). I would not assume that any old AC tach that fit in the hole would work. I think your best source of info is the AC tach you have that's working now.

[Dan J]

I also worked on Detroit's in my youth. Loved them
I agree with WD about the pick up point being off the cam gearing, though I can't remember if the cam is half crank speed as the engine is a 2 stroke.
A further complication could also be that on many of those marine applications one of the engines turns CCW.
I'd take out both tachs and make sure the cable turns the same direction if so send your new and old tachs to a good instrument guy and he should be able to match them

[drcoastline]

Widefifteen- I am using the good AC tach as reference. Unfortunately there isn't much info that is legible. I have found a few NOS tach's that appear to be the same. But I am not positive because I don't have any info to reference.

Dan- the Starboard (right) engine does turn counter clock wise.

[chocolatelab]

That is amazing. What a spectacular boat!!!!!!

[daepp]

Yes - that is a beautiful boat. May I ask what it is?