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In my workshop manual, it is stated that the maximum rebore allowed is +.50mm (+0.020"). This would take the capacity (assuming original bore size was the standard 74.70mm to about 2532cc.

 

I have a number of questions relating to this:

1. What limits the maximum rebore - is this just piston rings, or block strength? Classic Racecraft advertise a 2.7l engine ... how? (See p45 of TRAction 177). :0

2. What sort of performance enhancement would one achieve from a rebore? More torque, power, smoothness, flexibility?

3. What advantage do we get from higher compression etc. How are U.S. spec engines set up for less compression (piston heads?).

 

Answers on a postcard .... :D

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Piers, I can only answer your questions to my humble knowledge, an expert should correct me if I'm wrong.

The bore limitation is given by thickness of liners, as soon as the walls become too thin they will end up in barrel shape and blow or crack. Cooling is also involved here. For racing application where liners are often replaced every season there are less problem so far. Note that racing engines are not made for longevity, but for performance.

As the Americans say Nothing goes over Cubic Inches.

Thats why the big engines have a lot of torque and still run smooth on low grade fuel.

As far as I know high compression is the enemy of low grade fuel, this is one of the problems we are now faced with.

U.S. specification engines had different heads,and nowadays

they shave them off to have more compression.

For our cars here comes the next problem, it's like brake disks you can't shave off indefinitely.

jean

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You can bore to greater oversizes than +20 thou.

Standard oversize pistons only go so far but you can get bigger non-standard pistons from specialists such as Classic racecraft & others that can be fitted if suitably modified.

Ultimately you are limited by the proximity of number 3 & 4 bores to each other.

Oversize pistons can be useful in rescuing what might have otherwise been a scrap block. Generally it doesn't make a huge difference to a road car but every little counts on a race car. Unless you go to extremes reliability shouldn't be compromised.

You can certainly go up to 2.6L & still use a standard head gasket. Beyond that & it gets tricky & more expensive still.

These things do alter compression ratio's so if you do have to fit big pistons don't skim your head too far. (I think this is where the US cars achieved their lower compression ratio - but I'm no expert on US spec)

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Triumph only ever listed pistons at +0.020" oversize for the 6-cylinder engine, probably because they knew that practically all the cars they built would be scrapped through corrosion long before their engines needed a second rebore..  Enough of the cynicism, on with topic: Hepolite listed pistons at +020, +030, +040, and +060. The latter size became obsolete some years ago, leaving the other 3 sizes still available. +060 is now available in a "repro" spec - still perfectly good quality for a cast piston. Forged pistons are available in Std, +040, +080,

and 77.5mm(+112). 77.5 mm bore gives 2.7l leaving plenty of wall thickness, but does require a special head gasket.

 Grunt is added not just by cc alone but by the unshrouding effect of moving the bore wall away from the inlet valve, allowing better flow of the inlet charge. A 2.7l engine with a well-worked head is quite entertaining..

 I don't quite understand Jean's remarks about liner replacement - some confusion here with 4-cylinder engines?

Longevity is not an issue as long as the engine is fed plenty of clean oil at the right temperature, the right quality and quantity of fuel, and is properly cooled. Clearances are also critical.

 Don't believe anything Andy Moltu says about his driving skills- I supplied his special 97mm. stroke crank and 80mm.

pistons (ex Fiat 500 - it's true, and they do actually work if you're lucky with the block). Work out that capacity if you dare. Next daft project: fit tuned Stag engine into TR6 - has

anyone ever bothered?                                                     (N.B:Some of the previous paragraph may not necessarily be true..)

 Finally: a rebore will not give a noticeable gain unless 1. The engine is completely goosed  2. The rebore gives a substantial increase in size.  Many engines are happy with a ridge ream,

hone & rering, though watch out for cracks in the gudgeon pin bores of OE pistons.

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Sorry Jon for the misunderstanding,your remark is well placed as part of my text vanished when re-editing.

My remarks about liners were only the second part of my text and were meant to visualize the general effect of enlarging engine bore and that the limitation on reboring is wall-thickness available in the engine block, the remarks were not valid for a specific car, and certainly not for a TR5. Note that pistons and rings can be supplied by specialized firms in any size, to your own individual specifications.

So Andy got some Italian blood in his TR6, nice to hear because my TR6 engine got valves from a Lancia Turbo Diesel. The Italian Connection I would say.

Regards - Jean

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I have just has a total engine rebuild on my TR5. The new capacity is 2700cc. Mazda pistons and a 125x cam giving an estimated 190BHP. Reworked head and a set of s/s extractors and an up rated oil pump. This car has exceeded my performance expectations. The cost of this rebuild here in Australia was around 1200.00 pounds fitted..

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  • 17 years later...

Hello,I have just come across this discussion.I am going to add a  bit.Hope it works! Just started to rebuild an old US spec.TR6 motor and am looking for some reasonably priced and available pistons.What are the Mazda pistons if you don't mind? I think I also asked Jon Wood about his crank dampers but have lost the connection to him.regards,Michael.

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On 5/25/2019 at 1:00 PM, michaeljf said:

What are the Mazda pistons

Mazda or VW pistons. Very popular. I am installing VW 77mm Pistons. Pistons are hypereutectic, somewhat between casting and forging. Recommend CR 10.5.

But either engine block, pistons and/or rods require modifications! 

Jochem

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Hello Jochem. I would like to what pistons so I can buy a set to put in my TR. And the modifications if possible ? One block with damaged 2.7 bore and one with standard bore and Kastner S4 cam etc. Regards,Michael.

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