SME Tonearms

The SME tonearm presents a challenge.  

1. SME tonearm only have 2 small bore holes at the headshell for attaching the cartridge.  Unlike some other arms which give a slot at the headshell SME tonearms do not allow  "Effective Length" adjustment.  

2. Arc-type Protractor, which is a design used by my Best Tractor, gives a prescribed "Effective Length" arc for the user to get the exact measure. This is the most critical part of the Tractor.  

This means if the tonearm does not allow effective length adjustment then the Tractor will not do its job.  

From Picture above, the left headshell is SME like design. In contrast the right headshell does have a slot which allows for Effective Length adjustment.

The following gives a more detail explanation of the difference between a Fixed Headshell and a Slot-type tonearm:


Different cartridges has different Mounting Hole to Stylus Tip [MHST] distance. It is obvious from the above that the Effective Length of a SME tonearm is FIXED in relation to the cartridge to be mount on it.

Hence for a Best Tractor request on SME tonearms we need the MHST distance of your cartridge. Having this information we can determine the Effective Length. By formula we can then find out the appropriate Mounting Distance for this tonearm/cartridge combination and come up with a Best Tractor. Luckily, SME tonearms do allow Mounting Distance adjustment by providing a sliding base.

When you move the SME sliding base, you are altering the Mounting Distance and Overhang distance. But there is no change in Effective Length.

It is easy to measure this distance by yourself. Just detach the cartridge and measure it with a common transparent ruler. Take a digital picture and send it to us for reference. Examples:

Yes, you can measure it yourself just like I do.

For instance the clearaudio cartridge on the right picture get a 9.5mm MHST distance.


The cartridge on the left hand side is around 6.9mm MHST distance.

A Best Tractor prepared on this type of tonearm will be cartridge and tonearm specific. This means the Tractor may not fit other cartridges you would like to use in future.


Please do NOT use any caliper ruler to do this job. Cartridge body may have strong magnetic property that attracts any metal near it and you may damage the stylus when the tool is attracted.

A common ruler will do the job.

This is another example, MHST being 10.1mm.

  Ortofon has listed their cartridge dimensions for reference, please refer to the following link:

Ortofon Cartridge Dimensions


The following link gives a very good account of the issue:

SME V with Mint LP Tractor



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