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3000 BHP KB Block


Our machine shop is regularly asked to repair engine components in the belief that both the strength of the repair and the cost will be no problem. The following example is used to help explain the procedures, equipment and time, involved in one of the more complex repairs and to provide some discussion on the strength of this type of work.

The damaged component presented to us was a Keith Black big block Chrysler that had thrown a rod. For the benefit of those not familiar with the KB block, it is an after market aluminium cast block produced for the race scene and typically drives the 3000 bhp drag cars.

In analyzing the failure, a sequence of events appeared to have started at a rod journal at the point of a lubrication anomaly. One of the rod bolts then broke as the crank refused to stop during the resultant, but momentary, siezure at the journal. The rod then left the crank to continue turning whilst the rod accompanied the piston heading towards the combustion chamber. Not, however, fast enough to avoid being hit as the crank came around again to deliver a severe blow to the rod breaking it off the piston and wiping out both sides of the block taking the pan rail and main oil gallery with it.

Considering the 9000 RPM of this engine, it is understandable that considerable energy was involved in this event.

On closer inspection, severe fretting at the rear main bearing cap suggested vibration was going on in this vehicle. The team explained that their clutch and tyre set up had yet to be fine tuned and that the driver had actually had his teeth chipped during one of these vibration episodes. This vibration is also transferred into the rear of the crankshaft and then through the bearings, their bearing saddles, or housings, until some dampening occurs via the chassis etc. or ceases because the tyres and clutch have got the drive mechanism in harmony.

Returning to the block damage, the rear main bearing had spun in the tunnel during this vibration problem and caused a lot of damage. It is most probable that the rear main bearing developed excessive clearance during the fretting which bled oil pressure away from the rod journal that had then eventually blown. Closer inspection of the bearing saddles identified fine cracking from the corners radiating out into the webs of the block. Again, this is evidence of the energy involved in such a vehicle and the team’s very considerable skills in harnessing this to get that safe and fast time when they go racing.

The repair was not economic as the following process explains; that is, unless our Machine Shope Team was interested in a challenge. So, leaving the economics aside, the repairs to the block proceeded as follows:

Material required: another rear main cap; suitable aluminium plate to fill in the windows in the side of the block; some rectangular aluminium from which new pan rail sections could be made; some 100mm round from which to turn a half sleeve to repair the rear main block damage, and other minor items like alloy tapered plugs and screws.

Equipment required: Welding was by the TIG method; a lathe, a milling machine, line boring and vertical boring machines and drilling and engineer’s files and scrapers were all used to reclaim the damaged block.

Labour time required: The time taken for each stage of the repair is covered below.

Labour skills required: You work that one out after reading what had to be done to extend the life of this block to eventually run an extra 500 BHP after a bigger PSI blower was fitted and still running two seasons later. This isn’t meant to suggest that the repaired block was now stronger. Not at all. All we did was to discuss the cause, vibration,
( another subject ) and to reconstruct the block using the best methods and materials available to us. But this type of repair will not be as strong as a new block.


Dimensioned drawings were first made of the block as a reference point to return to after all of the welding and machining.

The pan rail of the block was then milled out so that all of the damaged section was gone and a platform established onto which the new pan rail sections could be welded. The holes (windows) in both sides of the block were burred out using an air grinder and carbide burrs to shape the windows into rectangular shape without any square corners. The welding process was expected to create a lot of stress risers without us adding more by way of square corners that cracking could start from.

The new pan rail sections were then milled into shape and made ready for the welding stage. Two large windows were then made from the plate material and hand fitted into the damaged areas as close as could be achieved to minimize the welding.

The TIG welding process was addressed with considerable care so as not to create shrinkage cracking and stresses that could develop into serious trouble when the engine was in operation. During this repair, the missing main oil gallery was completely reconstructed by welding. Damage at the bottom of two sleeve parent bores was welded.
Final work required establishing new drilled and tapped holes for the sump bolts, re-machining of the weld repaired lower sleeve parent bores, and re-drilling of the main oil gallery.

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Welding and machining of the lower sleeve register damage as viewed from the damaged rear main. Held in hand is the lower rear main cap that was replaced. Note the heat marking.




The main bearing tunnels had serious problems. The stress cracking along the block at several of the cap saddles was carefully inspected to establish the end of each crack. Welding was not considered to be an effective and safe method due to the very high loadings along this structure. Instead a hole was neatly drilled and tapped at the very end of these cracks and a plug inserted with Loctite 680. This was seen to leave the valuable heat treatment strength, that welding would reduce, and move on with the belief that we were going to address the vibration problems that were causing this.

engines 027View of pan rail, “window”, and reclaimed oil gallery after welding and machining






The above fabrication, welding and machining consumed some thirty five man hours.
This left the damage in the block half of the rear main tunnel and the damaged rear main cap.

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Rear Main saddle repair method after fretting damage from the vibration.

Note the small aluminium pad being hand fitted into the recess, (left hand side) machined out of the bearing cap saddle to remove the fretted surface. Both sides had to be reclaimed. Welding was not the safe method at this very highly stressed point.



The block was next set up in the milling machine and the fretted saddles machined for a minimum clean up.

The block then set up in the line boring machine with the old damaged cap fitted and tensioned up. This tunnel was then opened up approximately 3/8” in diameter and the old damaged cap thrown aside. The saddle repair involved fitting a new section of material machined from the 100mm aluminium bar to create a half sleeve, fitted, dowelled and screwed into the saddle. This was then machined in alignment with the other tunnels in the block.

Like any main bearing cap from another block, it doesn’t fit other blocks. The factory process machines each block with its own set of caps and does not expect interchangability between blocks. The new cap was presented to the block and it fouled on one side and had a gap on the other. We had a sideways location problem on the studs.

The tight side of the cap was then milled to allow the cap to fit over the studs so as to assess the shape of the tunnel. The approach was to then machine the cap to locate it sideways and to close the tunnel up close enough to allow final sizing by hand methods. Hand work was selected over another line bore because the new cap was thin enough as it was; we needed maximum strength left.


engines 027One of the “window” repairs viewed from the crankcase. A good example of our in-house welding skills and facilities.






Initially, the tunnel was + 0.009” vertically, and sideways it was out by 0.155” (Quite a lot).
The end result had to achieve an interference (tight) fit of 0.002” sideways and within a few tenths of a thou in roundness and parallel along the centerline of the crankshaft line.
The sideways machining of the new cap obviously left a gap at the opposite side that was then neatly filled in by making a spacer pad in aluminium, dowelled and screwed into the new cap. Finishing off with hand scraping, this whole rear cap reconstruction took seventeen man hours but resulted in a very pleasing outcome.

Why bother ? Skills require practice and challenges. Success at this level makes the other work we do every day that much easier; that’s why.