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The ring gaps are an important thing to look at when assessing an engine for serviceability.  Excessive ring gaps will cause reduced compression, impair starting and running performances.

What is the ring gap?

The ring gap is the gap between the end of the rings.  This is measured with feeler gauges.
Tractor and engine manufacturers will provide tools to place the ring inside and then the ring gap can be measured.  The majority of tractor restorers will not have access to these tools as they are unlikely to be in production for older tractors.  The next best alternative is to place the ring into the bottom (unworn) portion of the bore (liner) and measure it there.  The bottom of the liner will not have had the piston contacting it and because it is unworn it will have the cross hatched honing marks clearly visible.
We can see the unworn portion on this liner.  The photo doesn’t show it well, but the cross hatching is visible.


What are the ring gap tolerances?

For original Ferguson/MF rings and liners…

Top ring gap  9 to 17 thou

Second and third ring gaps 9 to 14 thou

Duaflex oil scraper 18 to 37 thou (alternative oil scraper is 10 to 15 thou)

If you are measuring a genuine MF liner then the bore of the liner should be 3.3130 to 3.3145 ins.  The reason for the size difference in the original liners is that at that time they were unable to manufacture the liners accurately.  After manufacture they would measure them and then measure and match pistons to them in a pair.

New rings out of specification

We have had a few people email us to say that they have got new pistons, rings and liners and that they have measured them before fitting and found that they are out of spec.

These have all been manufactured by a non-genuine parts supply company and unfortunately some of these companies seem not to be able to manufacture parts to the necessary accuracy.

Typically the following ring gaps have been measured from these parts suppliers…

Top ring gap 23 thou

Second and third ring gaps 18 thou

Oil ring 15 thou

What to do?

Firstly measure either your tool or (more likely) the liner you are using to measure the ring gap in.  Check it is within spec.  If it is within spec and you are experiencing ring gaps which are too large then compression is likely to be compromised.

Ideally these ‘new’ components should be rejected and replacement parts sourced from another manufacturer.

Daryle has emailed to offer this tip…

Excellent advice on Liners, if I may offer a tip, is to keep your liners standing. Laying them on their side will often distort them.

Thanks Daryle, much appreciated – Steve and Ian.

Related posts:

  1. Assessing Cylinder Liners
  2. FE35 Piston Rings And Valve Stem Caps
  3. Piston Ring Compressor
  4. MF35 Engine Rebuild – Assessment