Crankshaft weight - does anyone know?

[Mark Serj]

Hi all - does anyone know the weight of the crank by chance as I don't have anything suitable to weigh mine with for courier purposes.

Thanks in advance

Regards

Mark

[len]

I can weigh one in about 10 mins for you.
Len

[len]

crank weighs 35lb
len

[Seaking]

crank weighs 35kg
len

I hope not that's seems a bit heavy to me. I would have thought more 3.5kg

Cheers Glenn

[len]

oops
35lbs !
len

[Guest]

That's about 16kg, which is light for a 3L engine.

However it's all compensated for by the humungous weight of flywheel and clutch cover.

[Mark Serj]

Thanks Guys, 16kg seems about right from feel.

Cheers

Mark

[dasadrew]

Bare crankshaft 15,66kg (approximately )

[Guest]

The rods and pistons are pretty heavy.
I would be curious if someone had the weights of them.

A 16kg crank works quite hard when swinging around 10kg of rods and pistons especially on the main bearings.

[len]

I can let you know tomorrow if you want as I have a spare engine stripped down.
Len

[dasadrew]

been there, done that

One Piston & Conrod complete with big end shells = 1.58 kg

[ATTACH=CONFIG]41210[/ATTACH]

[Guest]

One Piston & Conrod complete with big end shells = 1.58 kg

I hope you don't mind the tech conclusions that come from all of this.

They are seriously HEAVY, another 13kg,- so not far off the weight of the crank, with all those accelerations and decelerations to think of, so large crank counter weights needed.
+
Heavy rods and pistons = high piston skirt friction = high power losses.

This effectively also means double the 1.58kg on each crank pin (more than 3kg), which equates to twice the main bearing loading 1000rpm higher, compared with the Dolomite 16V.
(Effectively speaking the stag has a piston and rod weight + inertia rivalling that of a 1920s cast iron combo!)

The only reason they survive with the unhardened cranks, is the crank stroke is 14mm shorter, and it makes only 17bhp/piston instead of 33.
All of that reduces stress, but despite the improved engine balance of a V8, the main bearings No2 & No4 are HALF the width of the Dolomite too.

(so ONE PAIR of STAG pistons of 750cc makes the same power as ONE SINGLE 500cc Dolomite 16V piston).

To give you some handle on this, here's the rod and piston from the (famous) 1970s Spitfire engine which are known for going round to 7500-8500rpm and making 100bhp/L (if you know how), so roughly the same power as the Stag V8 from only 1300cc.

As you see the weight per crank pin is only just over 1kg, or around 4.2kg total, and the rod is a lot longer at 5 3/4" favouring high torque.
mk3_gt6.jpg

Here is the lightest rod available for that engine, the humble 1200 Herald rod from 1962, used in the Le Mans 24h cars.
rod_1200.jpg
Pauter steel rods come in at 100g lighter than this at 492g.


The piston and rod on the Daimler V8 is a lot lighter than Spitfire....probably around only 750g total, similar to a motorbike, ie. about HALF the weight of the STAG ones, which partially explains why it makes the same power as the Stag with 500cc less.
(It also has nice hemispherical ALL IDENTICAL combustion chambers with largish valves and pushrods which have lower valve train losses..)

Here's the later heavier rod combination fitted to the Mk4 Spitfire (common to the GT6 & TR6)
already 6% heavier which doesn't last, leading to the "let's throw a rod" syndrome on the long stroke 1500 engine.
1300_mk4_GT6_pist.jpg

I would imagine a set of Pauter steel rods for a Stag would weigh 25% less, & a forged piston only HALF the weight so probably more like 900g total, forcing you to remove about 2kg more from the crank counter weights....
It's quite possibly one of the largest single improvements that could be made to the engine, gaining at least another 25bhp in reduced piston skirt friction.

I know this was a vital change to the Dolomite engine/
Originally it had to have a special Freudenberg damper used to counter the huge crank vibrations caused by the heavy pistons and rods.

[jakesmig]

With a Stag engine rebuild costing circa £3K I would have thought the additional cost of sourcing the steel rods and forged pistons, and finding a competent machinist willing dedicate enough time to scalp loads of material off a crankshaft would deter most people, particularly for a possible modest gain of only 25 BHP. How much would it cost if it all went pear shaped and a replacement crank had to be sourced and the engine returned to standard spec? Far better begging Flying Farmer to put together fuel injection for you.

[Guest]

Well the steel rods are the easy bit.
USA made.
They are after all incredibly similar to the Dolomite ones.
For the pistons, in any case something will eventually have to be done, because there's nothing any good out there right now.
Sadly I don't have any forgings at this size, but when autumn-winter comes I would be happy to make a prototype.

Taking metal off the crank is pretty straight forward after a careful word with the crank balancers.
I have one who would be able to tell me immediately what the balance factor is for a given rod/piston combo.

The thing is, taking metal off is EASY.
Where it gets expensive is when the pistons are actually HEAVIER, cos you then have to weight the crank with tungsten.

Oh and btw 25bhp on a Stag engine is actually 20% more power in reality giving much longer life and considerably reduced fuel consumption.
If it's possible to add 25% here, 20% there, and 10% somewhere else, the combined effort is more than 50% more power.
It struck me, not too many people laugh at a N/A 3L engine making 200bhp even today.

[mole42]

It struck me, not too many people laugh at a N/A 3L engine making 200bhp even today.

Oh damn, I was thinking a Roots blower was the way to go.