Engine build spec?

[l10tus]

If using a 72.75mm crank in an 'L' block with forged Pistons, what length rods would be suitable?

Also, same question, but when using a 'Tall block' 711M ?

Are small end positions in Pistons generally the same - or do they always vary?

Replies / thoughts, with thanks.

Regards,

Phil.

[rgh0]

Hi Phil

With forged pistons in a standard 1500 block and standard crank stroke of 72.75 you can get the piston compression height made to match the rod length you want to use from standard 4.80 to BD length 5.23 if your a fan of long rods. Though with BD length rods it can be tricky to fit in the valve cutout for a big valve high lift cam set up as the ring groove get close to the valve cutout.

The same applies to a tall block but the compression heights do not need to be so low depending on your crank and rod combination.

regards
Rohan

[l10tus]

Hi Rohan,

Thanks for the reply.

I'm still in the process of building a spare engine(s).

My machine shop are treating it as an Hospital job!

I have a standard bore 711 Tall Block, a 618 'L' block, several sets of forges pistons and a 72.25 and 77. ?? Steel cranks.

I'm try in to find out which rod length I need to make up two steel e
Bottom ends suitable for the engines.

There must be a way to measure the set distance between crank and gudgen pin to arrive at the conrod length...... But the machine shop are struggling to work it out!

They've purchased some different length rods and are going through the process of testing to see if they are suitable.

I was hoping to work out the length mathematically and then try to source the correct length that way?

There must be loads of rods out there that would be suitable?

So, if you mount the crank, and pop a piston in the bore, surely it's possible to measure the length of rod required?

Or am I over simplifying it?.

Regards,

Phil.

[rgh0]

Hi Phil

The two most common crank strokes are 72.75mm used normally in the 1500 short block and 77.62mm normally used in the 1600 tall block. So it sounds like you have a set of blocks with matching cranks which is a good start. Does not sound like a hospital case to me

Any machine shop that's any good should be able to measure a crank stroke. If they cant I would gently suggest you find another machine shop that can measure it. If they cant measure it they cant build it. Yes you can easily measure stroke by fitting a crank and rod and piston to a block and measuring from top dead centre to bottom dead centre distance. But any machine shop can mount a crank in the centres of a lathe and rotate it to measure the stroke much quicker and easier

Rod length to use depends on the pistons you have and their compression height. Again the machine shop should be easily able to measure compression height of the pistons you have.

Normal rod length for a 1500 short block and standard short block 72.75mm stroke crank is 121.92mm /4.80 inches
Normal rod length for a 1600 tall clock with a standard tall block 77.62 stroke crank is 125.22 / 4.93 inches.

Are you building race engines or road engines as this affects the compression ratio and target max revs and thus what pistons and rods you may choose to use

it is easy to work out mathematically which length rods you need once you have the rest of the critical dimensions for the other engine components you already have.
i.e
crank stroke
actual block height from crank centreline to top
piston compression height ( details of any intruder volume)
head bowl volume
target compression ratio to match engine use and cams.


regards
Rohan

[oldchieft]

lotus-twincam-f39/twin-cam-block-height-same-1100-1300cc-xflow-bdh-t30842.html#p206424

Some info on block deck hight.

Jon the Chief

[l10tus]

Rohan, Jon,

Thanks for the replies and info, much appreciated.

Regards,

Phil.