The pinion is an integral part to any starter as it is responsible for transmitting all of the power into the engine. There are a number of factors that may require the spring starter pinion to be different compared to the pinion featured on the starter to be replaced. The following aims to explain the reasons behind this, and also the often misunderstood terminology ‘offset’.
It would be easy to think that all starter motors are the same for specific engines. However the type of starter will make a big difference to its characteristics. Electric motors will typically deliver less torque but more output turns than a spring powered starter motor. To optimise the output of our spring starters we often fit larger pinions than the starter it is replacing so that the number of revolutions is maximised whilst still delivering a high torque impulse which swiftly and efficiently starts the engine.
A larger pinion with more teeth needs more space which is why many of our starters are manufactured with an offset, unlike most electric starters. This means that the mounting face is machined eccentrically from the centre of the pinion so that when mounted on the engine, the pinion centre is moved away from the engine centre. This allows the larger pinion to mesh with
the ring gear just as smoothly as the replaced starter and explains why a Kineteco Spring starter may appear noticeably different.
There is an alternative reason that Kineteco may specify an offset spring starter. Our spring starters are slightly wider than most electric starters, and on some engines the engine block or other obstructions on may foul the spring starter. Adding an offset of a few mm can make the difference between being able to fit a proven reliable Kineteco Spring starter or not.
The tooth form of the pinion is a crucial factor regarding whether it will fit on an engine. There are two main systems of tooth form used on starter pinions, these are the imperial form ‘Diametral Pitch’ (DP) or the metric Module (MOD). These two systems govern the size of the gear teeth on the pinions.
Diametral Pitch form can be converted into Module and vice versa, therefore Kineteco may supply for example a MOD pinion spring starter that will fit in place of a FS electric starter. Below is a table showing conversion between Diametral Pitch and Module tooth form within the range generally used on ring gears of engines Kineteco commonly fit with Spring Starters:
| DP* | Mod |
|---|---|
| 6 | 4.23 |
| 7.26 | 3.50 |
| 7.82 | 3.25 |
| 8 | 3.18 |
| 8.47 | 3.00 |
| 9.24 | 2.75 |
| 10 | 2.54 |
| 10.16 | 2.50 |
| 11.29 | 2.25 |
| 12 | 2.12 |
| 12.7 | 2.00 |
| 14 | 1.81 |
| 14.51 | 1.75 |
| 16.93 | 1.50 |
*On many older starters the ‘Fellows Stub’ form is used to dimension the pinion. This is based on the Diametral Pitch system, but is represented by 2 numbers, for example 8/10. The 8 in the fraction represents the Diametral Pitch used for calculating the pitch diameter, thickness of and number of teeth of the pinion, and the 10 for calculating the height of the tooth. This
results in a ‘stub’ tooth form which improves the strength of the teeth. This is important in starters, particularly spring starters, where high forces are applied. Gear teeth in other applications are generally not made to this more rugged form as it has a negative impact on wear and noise, but since a starter motor runs for a (very!) short period of time in operation this is not a factor in the design.
Increasingly the Fellows Stub form is being phased out, and other specifications for strengthening the teeth are being used by manufacturers. Some common terminologies, all different way of expressing the over sizing of teeth of a gear, relative to standard forms, are as follows:
- Correction on Radius (Cor. On Rad.)
- ‘ 11 teeth on 12 tooth blank’
- Addendum Modification Coefficient (Add. Mod. Coeff)
- Profile Shift Coefficient
On non- Fellows Stub starter pinions, one of these terminologies will likely have been used to express an adjustment to the standard form to the gear manufacturer to enlarge and thus strengthen the gear teeth. Different amounts of correction can be applied to gear teeth, to increase the strength of the teeth and also to set the gear mesh at a certain centre distance dictated by other factors in the design such as clearance. This means that one oversize 13 tooth MOD 3 pinion for example may not be the same as another manufacturers! It also means a 12 or 14 tooth pinion could also be fitted at the same centre distance, with differing amounts of tooth profile correction.
Whilst these are technicalities of starter design, any adjustment to pinion dimensions must be allowed for when Kineteco specify a spring starter to suit a new engine, otherwise the gear mesh may be incorrect and reduce the life of the pinion. When we are approached to fit a starter to a new application, we will send a New Application Request Form. This will include a section for submitting dimensions of the existing pinion from which our experienced engineering team will be able to design and select the correct pinion to suit.
For further details on submitting information to ensure the correct spring starter is supplied, please see the info page ‘New Applications’.