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Cylinder Bore Size


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Hello All,

 

I don't have my Brown Bible to hand (a friend has borrowed it).

 

I recall the cylinder diameter is 74.7mm (I think?), does anyone know if there is a maximum permissible wear level before over-boring must be carried out?

 

Regards,

Richard.

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I might be showing my ignorance here but if the 1st oversize piston is +20 thou, that is only 0.5mm

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You're absolutely right Chris, I'm hoping someone can point me in the direction of oversized piston rings to counter only very minor bore wear.....I'm trying to keep an engine block standard.

 

Richard.

Hi Richard! There is usually a ridge at the top of the bore and some people call it the finger nail test not very accurate .But if it can stop your finger nail some people say it needs a re-bore. The correct way is to use a telescopic bore gauge. The bore does not wear round in shape but oval and off centre at the top and bottom. If you get a reading of + 0.010" wear, it should be re-bored. If fitting new rings in old bores, the bores s/b glaze busted to help them bed in.

 

Bruce,

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It is just a question what you are willing to tolerate.

I swapped to a modern hypereutectic piston with only 0.04mm clearance.

No blow by and no oil burning!

 

If you increase clearance the engine burns oil and pumps burnt air

out of the valve cover.

 

This is also an indication of loosing power.

 

A former limit was to lock the tube for the air to the air filter box and open the filler cap

and drop it on the hole. If the cap starts dancing when you pull the throttle it is time to refresh.

 

The official way ist that a clearance extension of 0.1mm is a limit

and the clearance of rings should not exceed 0.5mm

But why doing something on the engine if the owner feels fine with it?

Burning oil, especially when engine is pushed, is often related to valve guides.

 

I would have an eye on the oil pressure and often our TR6 have worn big end bearings.

I would not hesitate to change them to extend the lifetime of the engine.

That must be done before they fail and a slightly hammering noise after start

before the oil pressure comes up is also an indication.

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You don't really need oversized rings as the ring gap is not that critical (more critical that it's big enough). However, one trick is to buy a +0.020" set and gap them to suit the bore.

 

Bores tend to wear oval (more wear on the thrust side) and the degree of ovality is at least as important as the overall wear.

 

If just replacing rings then (as already mentioned) the bores need to be properly honed so they bed in and any wear lip dressed to reduce the risk of it breaking the top ring. Though if there is an appreciable wear lip you should probably be re-boring anyway.

 

Nick

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The correct way is to use a telescopic bore

It is easy enough to knock up a bore gauege.

 

2 x 1/4 unf bolts

1 x 1/2 steel round bar - drill and tap this for the 1/4unf bolts

 

Cut the heads off the bolts.

Insert the bolts fully into the round steel bar

Cut the bolts so that they just fit into the bore (a 1/8 " gap would be fine.

Unscrew one of the bolts to fit nicely against the bore wall.

 

Measure with a 6" vernier gauge.

 

It would be usefull to have lock nuts on the bolts

Round off the ends of the bolts.

 

Roger

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Per recommendations on this forum, I bought a bore gauge from Dasqua, 80 euros.

Perfect instrument at a 3rd of the price of a similar Mitutoyo.

Off course you need a micrometer to set the bore gauge.

Recommended.

Waldi

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I measured my bores (std size) and pistons.

The clearances were just within the limits.

I decided to order bew piston rings, do a hone.

I had no edge in the bores.

Be sure to do everything by the book, so check ring gaps (at tge bottom, where it is narrower) verical clearance in the grooves, etc.

Waldi

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