The exceptional failure of some big end pins used in TKM engines run on the British Championship control fuel has been a major source of concern to Alan Turney at Tal-Ko, the UK’s only manufacturer of kart race engines with 50 years expertise within the sport.

As an immediate step to try to pinpoint the cause of the failures, sample worn pins were sent to a highly respected and qualified metallurgist for full analysis. By way of underlining the fact that these pins themselves are not the cause of the failures we give in full below the findings of that analysis.

While the detail may not mean a lot to those without the appropriate expertise, the highlighted conclusions make it clear that they are of high quality material and to the correct hardness and case depth. The cause of the exceptional excess wear is indicated as breakdown in the lubrication film.

Says Alan: “This is an unprecedented problem and over the many years we have designed, built and raced engines we have become very skilled at pinpointing the cause of failures.

“To get failures like this in the middle of a batch of components that has been functioning correctly is totally abnormal.”

This is the report:

G. & J. Castle           Consulting Metallurgists



54 Sunderland Road





Report no. C79/1

For the attention of Mr. Alan Turney

Metallurgical examination of Big End Pins showing excessive wear.

This investigation was commissioned because recent examples of Big End Pins had shown excessive wear in one local highly stressed area. As far as could be ascertained there had been no changes to production procedures which could have been responsible for this effect. The object of this examination was to examine the worn Pins to see if there were any material or heat treatment features which could be responsible.

In addition, it was required to determine the actual failure type and what could be responsible for this effect.

All the pins submitted were examined visually, and under a low power microscope, then sample

Pins were hardness tested on the end faces using a Vickers hardness tester and converting values to hardness Rockwell “C”. Sections were cut across the pin diameter so as to include part of the worn section.

Sections were also taken to include the end faces so that an accurate measurement of the case depth achieved by the heat treater could be made.

These sections were mounted in a thermosetting resin and polished for microscopic examination.

Core hardness measurements were taken on these sections using a Vickers hardness tester, and results converted to hardness Rockwell “C”. Spot, spark and Metascop tests were conducted to determine the material type, and photographs were taken of any salient points.



Results of tests.

Visual examination showed that there was a band of surface roughness present on all pins. This band extended around part of the diameter and was of limited width. Under a low power microscope there appeared to be a series of pits/depressions in the surface. A typical example is shown in photograph A.

Surface hardness = 746 – 763 H.V. = 62 – 62.5 Rockwell C Specification 62 – 64 Rockwell C.

Core hardness = 429 – 432 H.V. = 43 Rockwell C . This value is not specified but would

normally be expected to be within the range of 40 - 45 Rockwell C.

Case Depth 0.040” inches.     Specification 0.040 – 0.045” inches.

Spot, spark and Metascop tests confirmed material to be EN36 type.

Microscopic examination of the worn area showed a series of pits in the surface associated with some very fine fatigue cracks as shown in photograph B.

In the etched state the core structure of the pin consisted of low Carbon martensite as shown in photograph C. The case structure consisted of fine low tempered martensite with no evidence of any retained austenite as shown in photograph D. The non-metallic inclusion content was found to be very low on both sections, indicating that the steel used in manufacture was very clean and of good quality.


Tests have established that these pins have been made from good quality EN36 type steel.

The standard of heat treatment was also found to be good with all parameters within specification and in good agreement with the heat treaters test certificate.

This would indicate that the reason for this excessive local wear is not connected with any material or heat treatment failures.

From the evidence available it would seem that the boundary film layer between the two surfaces is breaking down allowing metal to metal contact.

Under these conditions contact fatigue can occur which causes small fragments to become detached leading to a rough pitted surface.

Why the boundary film is breaking down is the key to determining the root cause of this problem, and will clearly need further investigation of the full system.

G.  M.  Castle