1275 Marina engine rebuilt and improved

[MMT1968]

Hi,

As I wrote in another relating post I have rebuilt a 1275 Marina engine and tuned it the classic way. After running in I drove the car to a company for dyno testing and adjusting the needles of the twin HS4 carburettors.
Unfortunately the guy at the dyno missed to switch on the air fan while running a test and the cylinder head gasket blew away. The piston in no 3 was badly overheatet and scratched the cylinder wall. The debris from that damaged the main bearings as well as the conrod big end bearings. So the engine needs a complete rebuild.
As new Pistons are necessary I decided to buy Omega forged pistons and switch to 18mm floating pins mainly to reduce the weight of the oscillating mass. But! My old conrods are balanced well and equal in weight but really heavy. Their weight is 743 gram each and they carry a pressed in 20mm pin. Much too heavy as I decided. So I want to use lighter conrods. Arrow conrods weigh about 580g but are expensive, MED sells the same conrods much cheaper but these conrods are only sold for a 1,625 crank. The Marina engine has a 1,750 crank which makes it much stronger and I wanted to keep that. So I decided to have a set of very light conrods produced to measure at Robson engineering https://www.connectingrods.co.uk/
Surprisingly this is not that expensive as I thought. The price is in the middle between MED an Arrow. The conrods I have ordered weigh less than 530 gram each and the 18mm pin is about 30% less in weight than the 20mm pin.
With this setup the total weight of the oszillating mass without the pistons is now reduced massively about 964 gram. That‘s a lot! The rebuild is not finished yet but I am sure the engine will run much better afterwards.

Chris

[rocco]

Woah, that's bad luck. Building a nice new engine only to have it blow up during a dyno test.

What kind of figures are you expecting to see from the new and improved engine after rebuild?

[MMT1968]

Hi rocco,

Well, we had some 85 hp with incorrect needles and obviously not perfectly timed cam with standard gear. I‘ll now install an adjustable timing gear and guess that‘ll help to get some more power out of the 266 cam. All in all I expect something around 100 hp, as the head has been fully ported with 35/30 valves and so on. I had Omega die cast pistons installed to a rised compression ratio of 10,4:1. After having to plane Head and block and switch to forged pistons it will have 10,7 to 10,8 compession ratio. So I guess my expectations are relatively realistic and the engine will have nearly double the power of the original.
The positive effect of this situation is that with exeption of the conrods the cost for the rebuild goes on the testing shop.
But don’t worry, the rest of the car has been modified also to cope with the improved power of the engine. It has adjustable Spax shocks all around, disc brakes at the front tie rods (radius arms) at the rear. Anyway, a car like a Traveller of 850kg was never meant to run 170 km/h nor will I do. Even if improved how ever it is still a car from 1968, not a modern one containing all the safety features.

Chris

[helendam]

Geometry Dash Switching to Omega forged pistons and lighter conrods sounds like a great way to reduce reciprocating mass, which should help with both throttle response and engine longevity at high RPMs. The move to 18mm floating pins makes sense for further weight savings.

[MMT1968]

The weight of the reciprocating mass has gone further down than expected as the new conrods weigh just 473g each. In total including cons, pistons and pins the weight is 1,430kg less than the original parts. Thats a lot. Additionally the adjustable vernier drive saves 100g.

Timing the cam with the adjustable vernier drive showed a mistake of the original marks of nearly 5mm. The correct TDC is shown in the picture by the center of the white mark. That makes clear that timing the cam is not wasted time.

Chris

[philthehill]

It is a fallacy to think that because the Marina crankshaft journals are of a larger diameter it make the crankshaft stronger.
The 1.625 dia journal from the 1275cc Midget/Sprite crankshaft is made from much better steel. The Marina/Ital crankshaft is made from a lesser grade steel and later crankshafts were made of even lesser grade steel.
BL had to make the Marina/Ital crankshaft journals bigger because their crankshafts kept breaking.
Because of the problems BL had with the Marina/Ital crankshafts I would not even consider using a Marina/Ital crankshaft in a tuned 1275cc 'A' Series engine.
I have a fully reworked 1275cc Midget/Sprite crankshaft fitted to my 1380cc 'A' Series.
IMHO having special rods made for the Marina/Ital crankshaft is a waste of money. I consider that you would have been better off fitting a reworked 1275cc Midget/Sprite crankshaft with 1275cc Cooper S conrods.
Only if you had the Marina/Ital crankshaft reground to give extra stroke and fully reworked then the use of special conrods could be justified.

[MMT1968]

Hi Phil,

„Only if you had the Marina/Ital crankshaft reground to give extra stroke and fully reworked then the use of special conrods could be justified.“

That‘s what I did.

Chris

[philthehill]

Thank you for clarifying the use of special rods and that you have had the Marina/Ital crankshaft fully reworked.
What size big end journal did you end up with as I understand that you cannot retain the 1.75" big end when you off-set grind the crankshaft journal but you can off-set grind to bring the journal to 1.625". Whilst the stroke is marginally lengthened every little helps as they say.
Phil

[MMT1968]

Hi Phil,

We ended up at 1.675 and lengthened the conrods to correct the pistons standing 0,75mm below block surface. They are now even. That has rised the compression ratio also to now 10.75:1.
We will see if it lasts and how much hp will it produce at last. The much lighter internals will not produce a single hp more but give a better throttle response. We hve balanced the crank, flywheel, clutch and front damper carefully and each one on it’s own. At the moment the engine runs soft and smooth with much faster increasing revs than before. That’s what I was aming at. It must now be running in carefully and after that we‘ll try another dyno test.