Cosworth Engine Failure

Published by pgriffin on

After an engine failure on an Escort Cosworth it needed to be stripped down to find out what went wrong, the findings make for a warn to others on how much damage can be caused by just one small part failing, and the importance of quality fasteners and meticulous assembly.


The engine had been rebuilt with low compression forged piston, freshly mapped to remove lean burning problems that coursed the failure of the previous engine. The car was being used on a spirited drive when a small pop was heard followed by a loss of power. The car stopped and recovered back to its home where it underwent the usual hopeful inspection of checking the spark plugs, distributer and all the other simple check in a hope that it was an easy fix. With nothing obvious showing itself a compression test was done which confirmed something more serious as piston 2 had zero compression and further checks showed no movement of the piston at all.

The strip down

With the diagnosis terminal the engine bay was stripped and the engine lifted out and mounted on the engine stand for strip down.Damaged Cosworth cylinder block

After dropping the oil from the engine the head was removed and the block was turned upside down to remove the sump only to be greeted by a pan full of metal and daylight shining though the side of the block, it didn’t take many seconds to notice what the problem was, the conrod and big end of No.2 cylinder had been destroyed.

Broken Parts.

Pair of Broken / Snapped Big End Bolts


  • Big end bolt with obvious signs of tension failure, indicated by the pronounced necking and no bending.


  • Big end bolt with no necking but bent head and torn grain structure indicative of sheer failure.


Broken con rod still in pistion

  • A connecting rod still attached to the piston with missing big end cap


  • Big end cap, squashed and deformed with both treaded bolt sections still installed.Engine block with bulge in side of casing and damage to bottom of the cylinder 2 chamber.


What happened?

Having collected all the smashed pieces of metal together and identifying each part it soon became apparent what had happened.

The primary failure was a big end bearing bolt. One of the bolts had failed in tension and one of the bolts appeared to have failed in shear. It will have been the initial tension failure that has set off a chain reaction which ultimately destroyed the bottom end of the engine.

From looking at all the broken pieces of metal leads to one conclusion for the sequence of events that would result in producing such a collection a broken parts.

Broken Bolt Showing Necking at brake point

Broken Bolt Showing Necking at brake point

Once the first bolt started to neck or elongate the clamping force will have reduced between the connecting rod and the bearing end cap resulting in the big end becoming loose this in turn further increased the load on the bolt due to impact loading from the now loose fitting big end. As the bolt weakens further the necking gets more pronounced until the cross sectional area reduces to such a size that the bolt fails.

Once the first bolt had snapped the one side of the big end cap would have been unrestrained, which explains the deformed shape of the end cap as it gets bend by the forces in the reciprocating motion.

The cap is eventually bend to such an extent that the remaining bolt is unable to withstand the bending forces and snaps as the cap tries to separate from the rod. As soon as the second bolt snaps the con rod and piston are free from the crank.

This second bolt would have finally snapped at maximum load which is when the piston was at the top of its travel, and in this instance it appears to have been on a power stroke, as the piston has been pushed back down the bore. As the piston was still located within the bore it is likely that it hit the crank at the bottom of the bore, which would explain the 2 nicks out of the piston skirt, and the score marks on the crank, see figure 3 & 4.

When the big end journal continued to rotate it hit the now dangling connecting rod forcing it between the side of the block and the journal, squeezing the big end and ripping the connecting rod in two, as the rod appears to also show signs of necking as shown in Figure 6. This also explains the damaged block and the damage to the bottom of the cylinder bore.


A bolt tension failure would normally be expected in a high revving engine due to the increased load induced by the high piston speed, however this engine had a standard rev limit and lighter weight forged pistons which will have reduce the load and subsequent stress within the rods and bolts.

Damaged Big End BoltAfter further inspection of the rest of the bottom end, which were still assembled, showed that one of the rod bolts from cylinder 3 had a small defect in the shank, see Figure 7. This defect corresponds to the same position as the failure point of the other bolt. This would suggest that the failed bolt may have had a similar defect. A defect like this in the shank of the bolt would act as a stress concentrator dramatically reducing the strength of the bolt.

The line of the nick corresponds to the pitch of the threaded section, which suggests that the defection was either created at the time of manufacture, or that the damage occurred during assembly of the big ends, possibly due to misalignment during assembly, however it is impossible to tell what caused these defects as no records of the bolts are known and they may have previously been used prior to being installed in this build.

The origin of these bolts are unknown as there are no manufacturers markings on the head, or even bolt grade markings as required on bolts manufactured to British standards, due to this lack of information an appropriate torque cannot be determined. Torque is the most critical aspect of assembling bolted fasteners, both too much and not enough can cause catastrophic failure of the fastener.

How to prevent similar failure.

Using quality traceable components it a key facture in producing reliable products and assemblies. Spending hundreds of pounds on a set of low compression pistons is all good providing the same care if taken in selecting the appropriate ancillary components like bolts and gaskets. Trying to save a few pounds by reusing bolts or buying cheap replacements might seem like an attractive option as to many they are ‘just bolts’ however these bolts are what holds an engine together and a failure of just 1 bolt can cost a whole new engine.

Contrary to popular belief it is not better to select the highest grade of bolt available, as preload is the main requirement for determining the appropriate bolt grade, as a bolt should be torqued to 80% of it proof strength. This also means that the practice of ‘nipping’ the bolt further after its been torqued to the required level may damage the bolt due to over stressing and potentially course premature failure.

Other Areas of Note

It is also noted that No. 2 & 3 bearing journals, connecting rods and bolts have a blackened finish indicative to excessive heat. All 5 main journals have a smooth polished appearance and show no signs of lubrication or oil pressure problems and as the big end journals are lubricated from the main journal that would indicate that there should have been suitable oil at the big end journals. The most probable explanation for the appearance of the big end journal is that as cylinder 2 started to fail the movement of the big end will have caused the oil pressure to drop locally.

Damaged Cosworth Crankshaft

A bearing failure was ruled out due to the tension failure of the bolt and connecting rod, as a bearing failure would normally cause the crank and rod to seize and therefore exert and amount of bending for on the rod to cause it to fail.


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