Maximum Bore for Six Cylinder Jaguar Engine


I have been reading in a few manuals that the maximum bore for the 6 cylinder engine is +0.030.

***For example the workshop manual for XK 150 reads "Oversizes available +5, +10, +15, +20 and +0.030.

***Jaguar E Type Owners Workshop Manual; "Maximum rebore size +0.30 in (0.76 mm)

I have read many topics here as to boring/honing blocks to +040, +060 over and our vendors sell pistons +0.030, +0.040, +0.060. (Why not +0.050??) Apparently, bores over +0.030 are normal today and apparently are not problematic.

Is there a document from Jaguar stating it’s OK? Or is this something that…just happened? Curious as to just how this came about.

Back story; I have a block that has been bored out to +0.040 and at the time I was unsure if this is OK and the block is usable.

Dick Maury’s input here would really be useful.

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My experience is mostly with the six cylinder engine from the Standard era through the end of the Mark V. In those engines, I run a 70 thousandths-over cylinder bore with no issues. I have the impression the typical oversizes “allowed” were influenced by what parts supply chain could readily be supported. My 70 thou-over use custom pistons. I don’t know if anyone has ever done heat and mechanical structure analysis for oversize limits. And practical experience depending on application (racing, road use, etc) might have insights.


I have a 3.8 and a 4.2 that are both bored 0.030" over. No dramas at all.

One issue regarding boring 3.8’s and 4.2’s past the Jag published limit is that they’re factory sleeved so I’d guess Jag didn’t want you to compromise the integrity of the sleeve. Plus ‘7x block’ 4.2’s have 2 horizontal slots between the bores connecting the Left and Right sides of the water jackets that are only sealed by the sleeve - another reason to keep the sleeve material as thick as possible.

People have often bored 3.4 engines to 3.8 without failure. If it’s a numbers matching car I wouldn’t recommend it. Momo and Cunningham were known for that but not the first. It’s +.157 But that may have been before honing to final size to match the pistons.

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I ones quite inadvertently found out how far you can take a Datsun 1200 engine: I had obtained a set of high compression Datsun competition pistons for it, and I neglected to actually measure the pistons I just took the machine shop the box.

They bored it awrighty…And then it became a Datsun 1500!

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Thank you all for the reply. Good info. I would still like to know:
Why not 0.050" over pistons?
Where/how/when did the decision get made to go over the factory MAX of 0.030"? I think we will never know; )

On the plus side no one stated DO NOT GO OVER FACTORY! So my 0.040 over bored block is just fine.

Thanks again.


I think you shouldn’t worry at all.

Aske in another way, what could happen? Is another 0.12mm increase in radius much?

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My 120 came to me with it’s number matching 3.4 block bored to 3.8. It need boring again, so I had it sonic checked, and it was judged to have “enough meat”, so we added 0.020" to that. Runs fine.