ALLISON MARINE TRANSMISSION PARTS | M & MH+
What is the difference between an Allison Marine M versus Allison Marine MH transmission?
Both are built for marine use, and are very similar. The main difference is that the MH transmission if built for more heavy-duty applications and are generally larger in size.
What type of oil does this transmission take?
This engine will usually use a lubricating oil of SAE 40. For cooler conditions (below 32 degrees Fahrenheit) SAE 30 may be used.
Allison Marine M General Information
• This transmission uses 6 quarts of lubricating oil.
• The maximum horsepower is between 375 and 540 horsepower, measured at the shaft.
• This model weights about 350 pounds.
Allison Marine MH General Information
• This transmission uses between 6 to 6.5 quarts of oil. The deep profile uses the higher amount.
• This model weights about 480 pounds.
• The maximum shaft horsepower is 230 at 1800 RPM (For MH 25, 30, 37, 45), 460 at 2100 RPM (for MH 15, 20 and 25 crew boats) and between 600 to 720 at 2300 RPM for the other models.
Bearing Maintenance and Repair
The bearings are an integral part of the Allison transmission. Here are some key points to ensure that they last with extended use.
• Bearings should be washed in mineral spirits when possible. If they are filled with hardened grease, soak them in mineral spirits.
• Look for signs that the bearings need to be replaced. These include scored, scratched or otherwise damaged rollers or balls.
• Check for any stray metal particles. If you see any in the bearings, be sure to clean them thoroughly.
• When you get new bearings, keep them in the wrapper as long as possible. If you have to place them down, do so on a piece of paper that is free of lint, instead of on a work bench.
• When installing a bearing on a shaft, heat it to 200 degree Fahrenheit to expand for installation, by using a hotplate or an oil bath. This process should take less than 45 minutes.
Hydraulic Pump System
Of all of our Allison marine parts, one of the most popular is the hydraulic pump. This pump performs an essential function, keeping the transmission running smoothly. Here is an overview of the operation, to help with diagnosing trouble.
At the bottom of most Allison marine transmissions is the sump, where oil collects. This is then pulled up through the oil strainer, before it reaches the pump. It is then pushed through the filter, and then the cooler. The selector valve body at the top of the transmission then pushes it through to lubricate the plates, through an orifice at the top. The selector manages the flow of oil based on whether the unit is forward, reverse, or neutral. There is an orifice at the top of the transmission where the oil is dispersed into the unit. If you suspect that the oil is not circulating properly, as shown by low oil pressure, then check the oil cooler, suction line, flange and pump lines.
Here are some other details about the Allison hydraulic pump system:
• The type is positive displacement, gear pump
• Oil pressure should be 130 PSI minimum in forward gear at 1800 RPM. In reverse, fuel pressure should be at least 110 PSI at 1500 RPM.
• The oil is contained in the flywheel sump for M10 models and the oil pan for other models.
• Always check oil after the engine has been running for a few minutes, and when the transmission is around 180 to 200 degree Fahrenheit. Please note that if the power pack is at an angle, the full mark will actually be above that designated on the dipstick. Be sure to measure when it is full and mark the correct location on the dipstick.
• Low oil levels (below 6 quarts, or 6.5 quarts for deep profile models) can lead to aeration, as well as having too much oil in the sump.
• Aerated oil can have a thin, milky appearance. If the oil has been aerated, it can cause the transmission to overheat or operate irregularly.
• If the water temperature is below 50 degrees Fahrenheit (with SAE 40 oil) or below 32 degrees Fahrenheit (with SAE 30 or SAE 15W-40) then run the engine with the transmission in neutral for at least 20 minutes, or use heating equipment to reach a safe operating temperature.
• If oil pressure is low when the selector is in the forward position, check the forward piston seal rings to see if they need to be replaced. Be sure to also check the flywheel, in case there are loose or missing oil passage plugs. Check the emergency engagement bolts as well for any signs of oil leaks.
Overhaul an Allison Marine M Transmission
The overhaul kits are a very valuable addition to our Allison marine parts, and they come complete, ready to get your transmission back to running like new. In order to overhaul the unit, follow these general guidelines. As always, be sure to consult with an experienced mechanic if you need more advanced help.
During the overhaul, discard any gaskets and replace with new ones. If any washers, rings or similar are warped or damaged, be sure to replace them as well. Also, be advised that some seals should not be burned due to the risk of toxic gasses being released. In general, be sure to maintain a clean area, and due to the small amount of clearance between parts, be sure to handle all interior parts carefully so they are not bent or nicked. If you need to clean the interior of the unit, be sure to use mineral spirits. Compressed air can be used to dry, but be sure it is not used on the bearings. Check bores and passages for obstruction. Check mounting faces for scratches and foreign matter. Remove with a crocus cloth.
Step One: Secure a sling that can hold one-quarter of a ton, roughly 500 lbs. Bolts can be removed from the bearing retainer to attach a sling to it.
Step Two: Remove the oil gauge rod, if attached This will be on the lower right side when facing the nameplate.
Step Three: Remove the breather located directly in front of the name plate.
Step Four: If there is a selector valve, take out the four screws that hold it in place and remove the entire assembly.
Step Five: When looking at the gears, remove the input shaft snap ring, which is at the center of the shaft.
Step Six: The smallest gear, the reverse sun gear, can be removed along with the nearest washers, which include the internal-splined washer and two brass thrust washers.
Step Seven: The reverse planetary assembly can then be removed, along with the washer behind it. This is a slightly larger gear, which fits around the smaller sun gear.
Step Eight: Locate the three split pins and gently drive them in with a hammer until they are flush with the backplate. These pins will be located at the top, bottom left and bottom right side of the plate.
Step Nine: If you have the compressor tool, use it here. Otherwise, compress the backplate to relieve the pressure on the snap ring (the ring that goes along the outside of the backplate.)
Step Ten: Install lifting bolts into the reverse back plate. As it is removed, sixteen springs and three lock pins will fall free. Remove these, along with the three split pins.
Step Eleven: After the reverse clutch plate and ring assembly are removed, the reverse piston (the larger gear located behind them) can be removed in the same way, such as using lifting bolts.
Step Twelve: Remove the seal ring and expander from the piston and housing.
Step Thirteen: Put the transmission on wooden blocks so that the front facing down. (See MH section below for differences between the MH and the M transmission.)
Step Fourteen: Remove the twelve bolts and lockwashers from the oil pan to remove it, along with its gasket.
Step Fifteen: Remove the three bots and lock washer on the oil baffle, a cylindrical piece located in the front spanner unit.
Step Sixteen: Remove the cotter pin from the output flange nut, which is the pin in the center of the front.
Step Eighteen: Use chains, or something similar, to make sure the parts of the transmission do not rotate.
Step Nineteen: With the motion of the transmission secured, use a spanner wrench to remove the output flange nut.
Step Twenty: Remove the output flange, a plate with 6 holes in it on the top of the unit.
Step Twenty-one: Use an impact wrench to loosen the stalking on the front spanner nut. Even if it does not loosen, go ahead and remove the ten bolts and lock washers from the bearing retainer.
Step Twenty-two: Attempt to remove the front spanner nut. This can be done with a wrench and torque multiplier. If it is too difficult, this piece can be removed with a rotary file, but be sure to not harm the output shaft, and to clean up all filed materials.
Step Twenty-three: Reinstall the flange and output nut. Install threaded rods and jam nuts. Hold the nuts and turn the jam nuts to raise the shaft, and loosen the front spanner nut. Turn them a little further to remove the front spacer nut.
Step Twenty-four: Remove the reduction gear from the housing, followed by the inner race, rollers and outer race. The shims can also be removed.
Step Twenty-five: Finish removing the pinion gear and bearings (as an assembly) as well as any shims and spacers. Finally, remove the input shaft.
Allison Marine MH Overhaul
Listed here is the information for taking apart an Allison MH transmission to perform overhaul work. For the MH, follow steps 1-13 on the M section, because it is very similar. The rest of the process is outlined here:
Step Fourteen: Once the transmission is on its side, remove the twelve bolts and lock washers from the oil pan to remove it.
Step Fifteen: Locate the oil baffle and remove the three bolts holding it in place.
Step Sixteen: Use the oil port in the front of the housing to access the front spanner nut, and destake it by using a hammer to gently tap it.
Step Eighteen: Use a spanner wrench to prevent the front spanner nut from moving. Use a torque multiplier and spanner wrench to force the output shaft out.
Step Nineteen: Next de-stake the rear output nut, by placing a shop towel or something similar in the gear teeth to prevent rotation.
Step Twenty: Remove the 9 bolts and washers from the rear bearing retainer. Use jacking bolts to remove the bearing retainer.
Step Twenty-one: Remove the input shaft assembly.
Step Twenty-two: Remove the six bolts and washers from the lower bearing retainer, and remove them, with the gasket.
Step Twenty-three: Place the output flange and nut back on the unit. Use wood blocks to protect the reduction gear. Install all thread rods with nuts, and use them to raise the output flange. Remove the front roller bearing.
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