Mac saw engines

[Terry Bentley]

Mac Saws

Lets just jump right into the different models of saws that are compatible as kart engines.

In 1958 the first standard series saws appeared. All of these were 4.9 cubic inches with the same 2.125" bore and 1.375" stroke.

Two styles of crankshafts were available on them. For the direct drive models, they came with a stepped crankshaft. A 5/8 " diameter pto that steps down to a 7/16-20 left hand thread. The other style is a 9/16" pto that tapers down at a 10 degree angle with woodruff key slot and 3/8-24 right hand thread.

All of these blocks with removable cylinder heads measure approx 5 47/64" in height. The center line of crankshaft is 1-1/2" from bottom of cylinder block.
The center line of exhaust port opening is about 4.450" from bottom of block. Exhaust ports can vary in height +/- approximately 0.010". Each exhaust port is drilled out to 33/64". Intakes are drilled to 7/16".

The first of this series saws introduced were the models D-30, followed with the D-36. Unique to these particular saws was the recoil starter used. It is a pinion style gear driven to a ring cast on back side of flywheel. The pull start rope was on top of saw in the center behind gas tank.

   

The D36  saw came with all three exhaust ports, D30 had only the two outer ones. Both had only four intake ports drilled. They were lower horsepower than the current super series models that had been introduced two years earlier. There are more details and pics specific to these saws in the next post below.

The next newer models introduced were the 1-40, 1-50 and 1-60. Even tho they had different power outputs, all three of these power heads shared the same cylinder block. But with some differences as noted.

Like the model D-30, the 1-40 had only two exhaust and four intake ports drilled. These models which include 1-50, and 1-60 had the early casting that incorporated an automatic oiling system which proved to be unreliable and was discontinued. A steel baffle was fitted on the back side of crankcase. The baffle was installed in these newer models even tho the oiling system was not in use.

   

Here is a closeup of the baffle in bottom of crankcase.

Below is the same block without baffle. Notice the ledge at bottom of baffle recess

   

Tho it shouldnt really make much of a performance difference at the rpm these motors will run, there is definitely a potential for turbulence.

   

This is what the newer casting looks like without the recess for oiling baffle. A smooth transition feeding the back intake ports.

   

All of the 1-40, 1-41, 1-42, 1-43, 1-45, and 1-46 have this port layout. Only two exhaust and four of six intake ports are drilled. No third port induction on any of them.

   

Even tho the 1-40  came with the more conventional recoil starter they used existing inventory from the previous models, D-30/36 including the shroud and side cover. The flywheel with cast gear on backside was also used until stock was depleted. This cast gear can be cut off on a lathe and used as a standard flywheel.

   
   

All of the first generation standard series saws had cylinder head reversed with spark plug exposed on the exhaust side of block. This includes D-30, D-36, 1-40, 1-50 and 1-60.

The next years introduced in the lower horsepower models included 1-41, 1-42, 1-43, 1-45, and 1-46. Not much differences in these with exception to what is noted.

The 1-41 and newer models cylinder head was flipped around and no longer had spark plug exposed. There was now a rubber boot to access the spark plug. This new shroud and side cover had the earlier pinion style casting eliminated. This saw looks just like a kart style Mc-6 engine even tho it has a shorter stroke and not all of the ports drilled.

   

This is basically what the rest of these models looked like. Minor differences that dont affect power output.

During the production of the 1-42 model year, removable heads were replaced with a fixed head block. The intake passage was also changed up.

   

You can see only two of the three intake ports are drilled and now has a fixed head that is square instead of round. Also the newer intake opening. This is a transition stage for existing intake manifolds. It also allows for early and late model interchange of parts. With exception to a parts cleanup seen in some of the 1-45 models with removable heads, there was no more major changes.  

The next higher horsepower output saws included the 1-50 and 1-60. They shared the same cylinder block as the  1-40 model. Also with the same oil baffle as described earlier. Starting with these higher powered  saws all three exhaust ports were drilled. But only the four of six intakes were still utilized. Again no third port induction on these models.

   

The 1-50 and 1-60 were identical in power output, and had the spark plug exposed like the others. The differences between these two models includes crankshaft, shroud, side cover and drive type. The 1-50 used a step crankshaft and was a direct drive model.

   

The 1-51 had the now standard style shroud, side cover and spark plug location as with the newer models. These came with three exhaust ports and now all six intake ports drilled. But the third transfer was not utilized.

The 1-52 and 1-53 production blocks had changed to fixed head and newer style intake opening. The 1-53 was end of this model number series. Another change for these last two models, they now have ball bearings on the pto side crank like a Mc10. The block is also drilled for the third port induction. Also another difference is the stroke increased to 1.5", increasing displacement now to 5.3 cubic inches. Same as a Mc6 and Mc10.

   

The 1-60 did away with that earlier shroud and side cover, but still had the spark plug exposed on exhaust side of motor. It came with the ring gear flywheel as stock depleted. It has a tapered pto crankshaft with gear reduction unit and larger chain.

   


On these gear reduction models the recoil starter was on the pto side bolted to gearbox. It engaged directly with the clutch. This model still had only four of six intake ports drilled out.

The 1-61, 1-62, and 1-63 were basically the same with some improvements. All of the exhaust and now all six intake ports were drilled out, but still no third port induction on them. The same transition to fixed heads and newer intake opening as previously covered in the other models. The 1-63 was last in lineup.

   

Here is the 1-63 showing fixed head and newer intake opening. There was a transition to the fixed head during late production of the 1-62 model, even tho the IPL does not recognize this. The 1-62 block would have an early intake opening with removable head or newer style as pictured above with fixed head.

All of these saw power heads can easily be used for a clone Mc-5. With a crankshaft stroke increased from 1.375" to 1.500", you basically have a Mc-6. The missing ports will need to be drilled out to match kart engines. With the shroud and side cover from the 1-60 model saw, you can clone a nearly identical Mc-5 by using any of the cylinder blocks after ports are added. The 1-41, 1-42(with removable head)1-51, and 1-61 are exact same casting as Mc-5/6 with exception to missing ports. All of the parts are directly interchangeable between these series saw power heads.  Either with a stepped or taped crankshaft, you have plenty of options for a kart engine that will yield the same power output with a little work.

These particular model saws use either a Tillotson HL-63 or HL-87 carburetor. Some of the later models could have been equipped with the flat back Walbro too. An obvious difference between the two Tillotsons is the HL-87 isnt drilled for a choke assembly. It uses a plunger that covers opening of air horn, instead of shutter valve rotating on a shaft. The early D-30 used a magnesium Walbro, like on the Mc-10 kart engine. It is similar to the newer flatback models used on most of the early kart engines.

There are still plenty of these saw motors showing up all the time. Eventually the supply will dry up and no more to be had. Just something to think about in your quest for the ultimate Mac powered vintage kart.

More to come.