14 replies to this topic

[Java_Green]

A+ 1275 block. I don't have the pistons and need to sort out a proper compression ratio. Is there something like a trustworthy default deck height to start from when sorting out suitable components and machining?

[Spider]

Not really. Normally you'd do a dry assembly and measure everything.

[eric67]

I would suggest you work with a safe deck height of 5 thou for your CR calculations and then get your block machined to achieve it once you have measured it for real during your dry assembly.

[Java_Green]

If there is not a default deck height, it means that the CR must have varied a lot in production. Is that really true? So, all blocks ended up with at least 5 thou, is that how I should interpret it?

[imack]

They had way more than 5 thou, I've never measured one but I'd say more like 30 -40 thou

[stuart bowes]

If there is not a default deck height, it means that the CR must have varied a lot in production. Is that really true? So, all blocks ended up with at least 5 thou, is that how I should interpret it?

Presumably, as each individual engine was built, the measurements were taken on a case by case basis and heads / blocks were machined to achieve a certain CR, within an acceptable range of tolerance

Surely they couldn't have just assumed that the castings were accurate enough to put together straight away

[nicklouse]

If there is not a default deck height, it means that the CR must have varied a lot in production. Is that really true? So, all blocks ended up with at least 5 thou, is that how I should interpret it?

Yes there would have been a measurement to which the deck would have been machined to but this would not give you the deck height as there are still so many variables. 
 

which why people like AC Dodd provide undersize gudgeon pins so that you can put different piston and rod combinations together to get an even piston height.

[Icey]

They had way more than 5 thou, I've never measured one but I'd say more like 30 -40 thou

Both of the A+ 998s I’ve sent to AC have measured between 0.021-0.022” deck height.

[Shooter63]

Presumably, as each individual engine was built, the measurements were taken on a case by case basis and heads / blocks were machined to achieve a certain CR, within an acceptable range of tolerance

Surely they couldn't have just assumed that the castings were accurate enough to put together straight away[/quote]

You might want to measure the valve spring seat heights to see the type of tolerances that BMC/BL/Rover thought were ok

Shooter

[ACDodd]

Assemble it first and measure the deck height.

Ac

[sonscar]

I suspect that no individual engines were ever measured during production.There may have been some tests at various batch numbers but what to do it the last thousand motors varied from OK to excessive?measure them all?Scrap them all?Sell them all?They were not high performance motors,just my thoughts,Steve..

[Java_Green]

I think I have got the picture.... As I interpret it, order pistons based on "best guess" and adjust by machining deck height.

My base thought was to order pistons with suitable dish and skim as little as possible (=possible to skim in future) to get correct deck height and thereby CR.

Not exactly "deck height", but closely related.... Who has 70.60mm die casted pistons with different bowl depth(=dish)?

MS, MiniSport, MED and Swiftune seems to be rather thin on this one ...

[mini13]

Yeah, i think this is the case, at least some of the time.

I I seem to remember seeing a manual where it mentioned block grades, A, B &C, refering to crank bearing bore diameter, and also cranks, and they would match them to get better clearance, I think this was around the introduction of the A+, it may hae been a metro manual, but that seemed to go out the window. 

Certainly the factory wouldn't be trial building stuff as it would take too long, but simple go/no go gauges/ measurements would have been plausible for some stuff,

 

 

 

I suspect that no individual engines were ever measured during production.There may have been some tests at various batch numbers but what to do it the last thousand motors varied from OK to excessive?measure them all?Scrap them all?Sell them all?They were not high performance motors,just my thoughts,Steve..

 

[DeadSquare]

The "Datum" for BMC A series engines has always been the surface to which the sump / gearbox mates, and machined to this Datum is the position of the crankshaft.

 

The workshop manual states the crank throw and also the length between the centers of the conrod.  The compression height of the piston is not given, but is quoted elsewhere, ie: by manufactures or retailers.

 

By simple addition of these three figures, a theoretical deck height can be calculated. 

 

Don't forget to also include in the calculations, the difference between the thermal coefficient of expansion of the different metals, and incorporate a guess that will cover crankshaft whip and conrod stretch at high RPM.

 

In order to make quite sure that the piston doesn't come into contact with either the cylinder head or the valves, BMC also included a safety margin for good measure, so bear in mind that it is this safety margin that you plan to eliminate.

[Java_Green]

The "Datum" for BMC A series engines has always been the surface to which the sump / gearbox mates, and machined to this Datum is the position of the crankshaft.

The workshop manual states the crank throw and also the length between the centers of the conrod. The compression height of the piston is not given, but is quoted elsewhere, ie: by manufactures or retailers.

By simple addition of these three figures, a theoretical deck height can be calculated.

Don't forget to also include in the calculations, the difference between the thermal coefficient of expansion of the different metals, and incorporate a guess that will cover crankshaft whip and conrod stretch at high RPM.

In order to make quite sure that the piston doesn't come into contact with either the cylinder head or the valves, BMC also included a safety margin for good measure, so bear in mind that it is this safety margin that you plan to eliminate.

NOW WE ARE TALKING!!!
If the bottom of the block is the "datum plane", I assume there must be a block height measure, anyone? With block height measure I can calculate static deck height (as it is described it means without oil film distances in main bearings as well as in both ends of the conrod) It is a must that the safety margin covers thermal expansion coefficient differences and dynamic run out due to inertia)