Pressure fed 3 bearing cranks

[Jacob Fowkes]

I’m new to cars so go easy 
While waiting for a load of bits to turn up for my special I started work on my new engine. I got a nice crank case from forumn a member (cheers !) and set about making a new crank. Having drawin it all up on the computer and ordered my material I just wanted to work out how to route the oil to the big ends. Would it Be best to go all through the nose ? All through the centre bearing or abit of both. Well abit of both  is what’s I went for. To test the oiling system ive modified an old crank. Don’t worry I don’t plan on using this. There’s so much knowledge on here so I thought id get the ball rolling. My new crank has larger bearings and thicker Webs. To test it’s fit In the case I’m just 3d printing a life size model. Im sure a lot of you clever people are wondering why I don’t mod the case and fit a new 2brg crank. Well... my wallet and wheres the fun in that. Thanks so much to dave dye and Chris for the help. (The crank case has been line bored & stiffened.            
I plan to blank the ends of the holes and set it up in a case. To test what kind of flow rate pump I will need and what size I should drill the oil ways in my new crank. (The poor drawing shows the drilling)

[dickie65]

Hi Jacob.
Austin fed the centre main bearing on the three bearing pressure fed engine.
The crankcase already has the feed set up for it.
The flow will be more even from the centre of the crank.
A Dave Dye oil pump should be suitable and fit the case with minimal alteration.
I am running 60 psi in a two bearing engine. I suspect with centre feed you will not need such high pressure.
Make sure you get the centre bearing shell of the crank line bored with a the block you will be using bolted down and get some spare shells bored at the same time.
It is a good idea to fit O rings externally on the cam front bearing and fill in the oil groove or make a new bearing without the groove and fit a restriction in the camshaft feed. ( I use 1/16 reducing bush in the oil gallery )
It might be worth looking into narrowing the rods to enable a thicker section at the centre web and use rods without pinch bolts.

[Jacob Fowkes]

Thankyou, yes I plan to reduce the centre 2 rods to beef up the webs abit. Thankyou for getting back to me. I’ll get some more photos tomorrow of the mock up.

[Stuart Giles]

If you're looking at making a crank, it ,might be worth taking a look at a Reliant SV crank which has the same bore pitch as an A7 but has thicker centre webs by virtue of increasing the big end offset whilst using the same big end width as the A7.

If I was going to go down this route, as a start as well as increasing the big end offset, I would look at increasing the diameter of the centre main and making it narrower which would allow yet thicker centre webs on the crank. SV Reliant cranks in common with many other three bearing motors tend to fail on the centre web. In fact, while you're making a crank, why not increase the big end diameter too?

Some time ago I engineered and did trackside for a blown 3 bearing car. This used a Reliant SV crank and rods and had all 4 big ends lubed via the centre main, our experience was that at high revs an overbored A7 oil pump simply wasn't supplying enough volume/pressure to the bottom end. Centrifugal force is not helping the lubrication of numbers 1 and 4 big ends at high revs.

If I did this job again with a Reliant crank, I would seriously consider keeping the front and rear main shell style bearings and keep conventional big end lubrication with only big ends 2 and 3 being fed from the centre main. This is what Chris Gould (who pioneered the Reliant crank conversion) did for the 10,000 mile record attempt. AFAIK, Chris's (now road going Ulster Replica/special) still uses this setup. The only giveaway of the rather special bottom end is the complete lack of ball and roller bearing noise when the car is running.

[Jacob Fowkes]

Thankyou so much for commenting I knew there where some very  clever people on here. I’ve got a scrap reliant crank that is helping as an ideas supplier. Once it’s done I’ll post the 3d model.

[Bob Culver]

As a forging it is esp difficult even if you have an old skilled patient mate with forge and hammer and access to a suitable hunk of fancy steel...Are you planning to have an SGI crank cast? I suspect a nitrided 2 brg 1 1/2 would prove vastly more reliable and you could restore several other cars with the time saved.The 3 brg crankcase with the centre bearing dangling from flimsy mounting is fundamentally flawed and not worth entrusting an expensive crank to.

[Jacob Fowkes]

Thankyou bob that’s a good point

[Ruairidh Dunford]

These are the only photos I have showing the internals of the Grasshopper engine Dave Williams built:

   

   

   

[Jacob Fowkes]

Wow they’re some cool photos Thankyou for posting. Was it a stock crank that had been modified ?

[Ruairidh Dunford]

I do not know Jacob, sorry.