correct gear pressure

[hATTISFACTION]

My old 41 Hatteras with twin Disc gears had a gear pressure of 280lbs.
My newer 1976 42C has Allison gears,Type M, that only show 145 lbs of pressure. They run fine and when I had the engines surveyed no mention was made of any problems with gear pressure.Does anyone have any input on correct gear pressure for Allison Transmissions on JT671TI's ?
Also , I presume fluid level is checked with engines running ?? is this correct ??
thanks for any input.

[Genesis]

Allison Ms run with 120-150psi in neutral or forward, somewhat less in reverse. Oil level is checked with the gear HOT, in neutral, engine at idle. Use the same oil as in the mains (40wt straight grade) These gears' clutches will clamp fully with as little as 90psi, although anything below 120 or so is cause for an adjustment of the hydrico pump (which has an external adjustment for pressure.)

These gears are also of a "wet flywheel" design - if the oil turns black on you suspect the rear main engine seal is leaking, and engine oil is contaminating the gear. This is a real B#@## to fix, as you might imagine, as the gear has to be pulled to get to it. The seal itself is only about $30 - labor charges often exceed $2k!

Some of these gears had a problem with the upper bearing not getting sufficient oil flow. Miost of them out there nowdays have had the "fix kit" installed in them, or they would have gone "bang" by now. They're great gears and provide excellent service, although they're unbelieveably heavy. Their "downfall", if they have one, is that due to their design they cannot be fitted with trolling valves.

Caution: Oil level is critical in these gears! Overfilling them is very common as they are often checked when cold; when cold the level should be in the lower half to the halfway point of the "safe" area on the stick, NOT at the "full" line.

[Traveler 45C]

Same gears, 8V71's.

My oil level checks way above the full line when cold and normal when hot. What would cause this and is it a problem?

G

[hATTISFACTION]

Genesis , thanks for your response, I feel a bit better now about the readings I'm getting. Interestingly my oil level is the same as the previous post on this . That is they show way above the full mark when cold and then normal when hot/running. I found this out since when I filled the oil I did it cold and the gear was way down when I started the engine and only went into gear intermittedly. Worked fine when I topped off when hot with a few more quarts of oil..
Thanks again , I have learned a lot from your responses to myself and others you have answered.

[Traveler 45C]

Ditto!

[Orion]

Just were is the location of the pump and external adjustment for the pressure. Would be nice to know for reference.
Thanks Orion

[Genesis]

.... the oil level will normally read high. Depending on your pump/filter location, it may read WAY high as the filter may dump back into the gearcase when the engine is shut down!

This is one of the reasons you need to check it running.

The pump is on one of the PTOs on the rear of the engine. Follow the hoses. :D The oil path comes from the rear of the gear up through a strainer (it has three bolts - when you change the oil/filter, open that up and check to make sure there isn't any crud in it - the strainer pulls right out) then through the pump, filter, and gear oil cooler before going back to the gearbox itself on the top center. The usual OEM filter is a cartridge that goes inside a cannister with a drain plug on the bottom - some people have mounted a traditional "oil filter" style block instead. Six of one, half-dozen of the other if you ask me, but the cannisters are prone to leak if you don't change the seals with the filter. They ARE under quite a bit of pressure - far more than a regular engine oil filter.

Shifting is done with a selector valve that is on top of the gear and selects which clutch pack gets oil pressure. Right near there is where the pressure gauge tap is located; things are a bit tight in there with the cable, selector lever, and oil hose coming in all pretty much in the same place.

I've never seen one read high cold - but nothing really surprises me in that regard, as the plumbing for the filter and such is often changed by owners. The big deal is to check the oil level with the gear at normal operating temp, at idle, and in neutral. I personally kinda like the AC cartridge filter setup on mine and have kept it as it came...

[Genesis]

There is what looks like a "double long nut" on the side of the pump. The end comes off, and inside there is an adjustment screw for the pump pressure. What you're adjusting is the bypass valve in the pump.

[dshuman]

is located at the top rear of the engine, at the back of the blower (it's powered by the blower shaft).

[SKYCHENEY]

I know that this has been discussed before and I have read all of the archives regarding this topic, but......here is my problem:

I have Allison M20's and when they are in neutral, the shafts turn. Now I am not concerned about the stb gear as I can stop that shaft from turning with my hand. But, the port gear is another story. On the port side the shaft turns more and it causes the boat to be propelled forward slightly, enough that you have to keep a spring line on while at the dock. I cannot stop this shaft by hand and it seems to increase in speed as I advance the throttle. I noticed that a new pump was installed on this gear and I am wondering if that is where the problem lies. Could incorrect pump pressure be the cause? The repair records show a new clutch in this gear too.

I have run the boat for about 150 hours since I bought her and the condition has remained the same--no better, no worse. I am going to change oil this weekend in the gears and I will check the screens at that time. Any other suggestions?

[Genesis]

What is the oil PRESSURE?

Do not trust the electrical gauges - pull the sender and screw in a real gauge.

My guess is that the selector valve is leaking slightly (on the top of the gear, easily removed with a few bolts) or the clutch packs are damaged. The former is not hard to fix, the latter is another matter entirely.

[SKYCHENEY]

If it is the selector valve, can you get a rebuild kit for it or do you just put on a new valve?

[Genesis]

I'd probably take it apart and see if I could find a bad seal somewhere.... :)

I don't know offhand.... haven't had one fail on my gears yet :)

[Trojan]

Gear pressure on my 43dc with 671N DD runs at 300 psi constant. Bill

[Genesis]

Those are Twin Disc or ZFs - Allisons don't run that kind of pressure.

[Trojan]

I don't know. I will have to look. I don't know one from the other. I thought it was what was in the DD. manual that came with the boat. I will look for a plate today. Bill

[Genesis]

I suppose they could be Capitol's too (I don't know much about those gears - but I know POST used them on several of their boats with 6-71s.)

They definitely aren't Allisons.... ;)

[Nick]

Probably not Capitols. Our 37 has the 671TIs with Capitols. The gear pressure should be around 200psi to 210psi at normal operating speed. The pressure is manually adjustable at the gear pump.

Nick

[eze2bme]

My 1979 43DC has DD 671N's with Twin Disc MG506 trannys. Pressure runs at 360 psi.

[SKYCHENEY]

I have no service manual for my Allison M20's. I assume that this is not something that someone could just copy. If it is anything like my 92 series manual with 15 large sections in 2 binders, then that is way to large to bother with. So, if someone as an exta copy or knows of a good place to get one let me know. If I have to I suppose I could probably just order one from DD.

[Genesis]

I wish I had one.... they're VERY hard to find. If you come up with one please ping me - I'd definitely like to get my hands on that!

[Traveler 45C]

I’ve got one. I suppose I could copy and send it to you guys. It’s not that thick. Please send addresses and give me a couple of days.

[dwaynec]

I would like to get hold of a copy too. I will send you address info in a private message.
Thanks.
Dwayne

[Genesis]

PM Sent - thanks!

[Traveler 45C]

No problem. It is the least I can do after all of the help that I have received from you guys and this great forum.

[Trojan]

I have just found out that I have MG-506 Twin disc transmissions.The correct pressure is 300-320 psi @ 1800 rpm and 180 Deg's. Per the manual. Thanks guys. Bill

[eze2bme]

Trojan ... I have the TwinDisc MG506's also ... and my Hatteras Owner's Manual also says 300-320 psi. However ... I called TwinDisc and gave them my serial numbers and they found the original billing & material order info. Some of these 506's were 300-320 psi units and some were 350-370 psi units. Mine happened to be the 350-370 psi units althought the manual says differently. You can call TwinDisc at 262-638-4000 or email Rick Lund in tech support at lund.rick@twindisc.com and he can determine which units you have.

[Trojan]

Thanks I will do that. Bill

[dakrhon]

I have an '81 48MY with the same problem, port side shaft turns in neutral. Moving the shift lever on the gear, I can't find a neutral where the shaft stops spinning. Can't wait to hear the answer on this one.

[Genesis]

If they're Allison gears the selector is in the box on top of the gear where the shift shaft goes to. You'll note that there's a gear oil hose going to that valve - that's the inlet for the oil. The valve can leak and if it does it will cause the shafts to hunt in neutral. That valve can be removed from the top of the gear (it just unbolts) and I assume it can be overhauled at moderate to low expense, although I've never torn one down myself.

If its NOT the selector then it probably IS the clutch packs. That's expensive as the gear has to come off to get to them, and those Allisons are amazingly heavy BEASTS.

[SKYCHENEY]

If the shaft turns, but you can stop it with your hand as you can with my stb trans, then that is probably nothing to worry about. Many of us with Allisons have shafts that turn and it's no big deal. But, if your shaft cannot be stopped my grabbing on to it, then it may be worth looking into fixing it. My port shaft cannot be stopped and it does cause some problems when trying to set the hook, etc. I've been busy with other things, but I'm going to check out the selector valve next and then go from there. I'll let you know when I have a solution.

[SKYCHENEY]

Finally pulled the selector valve today. It is a very simple design with a seal on each end and a ported shaft. Unfortunately everything looked okay with the valve. I reinstalled it with a new gasket and the shaft turning continues. :(
Talked to the DD mechanic and he thinks that the forward clutch is not disengaging completely because it is warped, misaligned, or some of the twenty or so springs around it that release it are weak. He suggests I use it as is until this winter and then pull it apart. OUCH!

[Genesis]

Yeah, if the selector valve is ok then the problem is in the gear, and is PROBABLY in the release mechanism or the plate is warped..... unfortunately.

[SKYCHENEY]

Here is the update on my M20 that was turning in neutral. It was the port trans and it was turning in the forward direction so bad that you could not stop it with your hand. It caused a problem while docking or setting the hook. Well, we finally pulled it apart and found the problem. There are about 10 springs that separate the pressure plate on the forward clutch when you disengage it. All of those springs were compressed and would not allow the clutch to disengage completely. It would drag. When compared to the new springs the old ones were about a 1/4 inch shorter. So, the new springs will go in next Wednesday and we'll try to get her all put back together. Makes me wonder if the slow turning in neutral in the stbd trans is also a sign of weakening springs. It does not currently cause a problem and you can stop the shaft with your hand. I guess I'll know more after I get her in the water and try it. I'll also have to realign once we launch. I'll keep you posted.

[mike]

How many hours did you have on your transmission? Also suggest you check the clearance between the piston and the inner and outer seal. You should be able to easily insert a .005 feeler gauge around the parameter. Pick 4 points starting at 12:00, 3:00, 6:00 and 9:00 to check for the proper clearance. I found on one of my units that the piston inner seal surface was not concentric with the outside seal surface, which in turn was causing a binding situation, and hence poor release in neutral. Bottom line the piston was defective/out of spec from the factory and had to be replaced. FWIW given that you have the tranny out, now would be a good time to replace the FWD and Rev. pressure plate piston seals, pilot bearing seal and main seal, assuming they need it. You can also replace the clutch plates with the new Gortex versions. This can all be done in the boat. The reverse clutch plate is not too difficult to remove. Parts should cost under $1k, and a days labor.

[SKYCHENEY]

Mike,
Thanks for the advise. I will check the clearance on the piston seals. But here is the overall situation as I know it. I bought the boat 2 years ago and it now has about 3400hrs on it. The port trans was rebuilt just before I bought the boat and has only about 400 hours. The mechanic that did the work for the previous owner obviously did not replace the springs because if he had, then there would be no way that they would be compressed like that. Now I wonder just what he really DID do. I will inspect the entire unit more closely now that we have it apart and I may have to replace some of the other items that you mentioned depending on what we find. Thanks again for the help.

[mike]

Glad to be of help. Given I did my own rebuilds, I can assure you the units are simple to work on, and very robust. I have the Allison rebuild manual if you need additional info.

[SKYCHENEY]

Glad to be of help. Given I did my own rebuilds, I can assure you the units are simple to work on, and very robust. I have the Allison rebuild manual if you need additional info.

Rebuild manual, huh? I assume that is different than the service manual that I have. If so, I wouldn't mind getting a copy of that. I will look at mine on Wednesday when I am back on the boat to let you know what I have. I know it shows a schematic of all of the parts, but I don't remember any specific rebuilding instructions.

[mike]

The trans manual is separate from the engine manual. It is about 40 pages of step-by-step detail. I have posted pics on this sites tech section of a few views of the guts of the trans. I'll send you a copy of the manual if you like. Send a PM with contact info. FWIW I stripped mine down to the cases and did a complete rebuild. If needed, you can pull them out the fwd stateroom hatch, and therefore avoid tearing up your salon. They are heavy (aprox. 500+lbs) however my son and I manhandled them forward, and then had them hoisted out the hatch. If you need parts and info Acadian Marine Transmission is a good source. 21 Burgess Dr., Broussard, LA 70518 (337-837-3636) Brian Lemmon is a good resource. The treated me fairly.

[SKYCHENEY]

thanks for the offer. I have an Allison manual so it is probably the same as yours. I will keep you all posted on the progress on this.

[SKYCHENEY]

PROBLEM SOLVED :)

We splashed yesterday and I got to try her out. Both shafts still turn in neutral, but BOTH can now be stopped with your hand. It sure will be a joy to not have to worry about that creep anymore while docking or anchoring. Thanks to all for the help.

One thing to note, though: Don't stick your head too close to the prop while lapping it in. It cost me four stitches above the right eye yesterday (how stupid) :mad:

[Traveler 45C]

Sky, sorry to hear about you getting prop’d. What happened?

Do your shafts still turn in N after warm-up?

[SKYCHENEY]

I don't know if they turn in neutral after warm up. I just pulled her around from the lift and into the slip. I want to check the alignment before I really go out for a true shake-down.

RE: Getting "propped"
I was lapping in the props and I stuck my head forward to see how far onto the shaft the prop had moved while I was still turning it. Ooops.

[SKYCHENEY]

Do your shafts still turn in N after warm-up?

Okay, I now have an answer. Both shafts turn slowly in neutral when cold but they do not turn at all once they are warm. This makes me happy :)

[Traveler 45C]

That’s the way mine behave.

[MGMullendore]

I also have Twindisc MG-506's on a 1969 Hatteras TC where one operates at 295 psi and the other at 350 psi. I gave a call to TwinDisc and Rick Lund is still there and most helpful. My units were shipped with a design operating pressure od 300-320 psi and he is speculating that one has had a change in the regulating valve/orifice resulting in the higher operating pressure. If not, then the regulating valve is probably stuck.

[oscarvan]

Probably not a problem since many of the "players" are still here, but you DO know you're replying to a 10 year old thread right? :confused: