Pinch on main bearings

[Keith Clements]

I have had problems with oil pressure on the Javelin for a few years. I initially put it down to overheating.

The engine was rebuilt with new bearings, as the white metal ones had disintegrated. The cranshaft looked good and within wear tolerance (but what is thought to be the most before regrind?). Reconditioned heads and an oil cooler were fitted. Another radiator, although not new was fitted, it was cleaned out and flow checked though. That was two years ago.

Oil pressure still dropped; but once again overheating was suspected. On inspection the white metal bearings had broken up again; so last month new bearings, new oil pump and new radiator were fitted.

Still oil pressure dropped down from 50 psi cold and above 2500revs to less than 10psi on idle when warm. Car had only done ten miles; before engine once again stripped down. Bearings look good; but the polishing pattern on the bearings was not as even as I would have liked. There was not a continuous polishing around the circumference; but a sort of wave at the end of the shell where no polishing had taken place.

On assembly I always check the crankshaft can be turned easily and there is no slack and correct end float. There were no grumbling noises when running.

Examined the oil filter housing gasket and oil delivery pipe gasket, both OK.

Then checked the protrusion of the bearing shells above the crankcase. On one block they were level, on the other the total on both side was between 25 and 30 thou on all shells.

I tried another set of shells and they were around 20 thou.

Checked an old block that still had shells in. One side of the block had no protrusion, the other was around 20 thou.

Has anybody any comment on what the pinch should be?

I am planning to put in a new shaft and bearings; and if that does not work to change the block. Note one side of the block has been welded; so perhaps a crack has opened up.

Usually I check assembly with plastic gauges that are crushed on trial assembly. I must admit I did not do that last time; but did the previous time.

Any hints on assembly would be appreciated.

[Keith Clements]

how do you do an oil pressure test with the sump off?

[Robin Fairservice]

My Javelin does the same, but there is no overheating. On starting up I have 60 psi, but as the engine warms up it drops to about 10 psi.
When ever I get my rear main bearing and we can reassemble my spare engine it will go in and then we will try and see where the pressure goes in the first engine.

I will be interested to learn the answer to Keith's question.

[Keith Clements]

Unfortunately I do not have an oil testing pump. How about an air line and see if I can hear where it is gushing out?

[Keith Clements]

I will check the bolt. Must record on here what the correct length should be. Is it in the manual?

[Keith Andrews]

When ever building an engine it is good practice to replace the oil pump, water pump, and pull the header tanks off the radiator and clean out (not just flush)
Even if the oil pump is only a few months old....
Failures of any of these components can cause serious problems where one has to start again and rebuild almost from scratch.
We use a Zn based oil to bed the engine in for the 1st couple 100 miles then replace oil and filter.
Around most places in the world Zn based oils are now limitted to track/racing use only.
Within the 1st couple 100 miles one would be suprised at the amount of debris and metal that is deposted during the bedding in pocess. These can cause excessive wear and block or limit oil flow, and if in the oil pump limit its effectiveness dramatically, espec if the oil pump is worn slightly.

[Keith Clements]

Nobody has actually answered the question... what should be the pinch on the main bearing shells?

[Mike Allfrey]

Dear Keith,

My word, what a subject!

Where to really start is the main question here. I think I have read somewhere that the 'crush' on the main bearing shells should be no more than 0.0025". But I am not sure. Your measurement (0.025") seems to indicate that the crankcase joint has been tampered with (bodged) at some time to fix a different problem. Has the engine ever suffered a broken crankshaft?

I had one engine that did and the front two main bearing supports were distorted.

Another point, at what undersize are the white-metal shells. As the amount of undersize increases, so does the thickness of the white-metal layer. It is well known that a thin layer of white-metal gives vastly better performance than a thick layer.

I would assemble the two crankcase halves and have the main bearing tunnel measured accurately. That could tell you heaps.

Another point to watch for is whether the crankcase tie studs and bolts have ever been over tightened?

Regards,

Mike A.

[Mike Allfrey]

Dear Keith,

Sorry, I have some more comments:

It should be noted that the crankcase joint line may not be the centreline of the main bearing bores and camshaft bearing bores.

Caterpillar dealers should have an engine oil pressurizing kit. The Caterpillar 3208 engine must be pressurized before initial starting after engine overhaul.

Reference Pat Lockyer's comment about attaching a pump to the flexible oil pressure hose. I would advocate having a spare union, that screws into the rear timing cover, that has had its restrictor drilling opened out. The small bore stops violent fluctuations of oil pressure at the gauge.

For the interest of others involved in this discussion, the main bearing bores in the assembled (torqued) crankcase should be 2.3950" and the main bearing running clearances are as follows:

Front and centre 0.000" to 0.002"
Rear Main 0.0005" to 0.0015"

My recent engines have had their crankshafts ground to suit the installed and torqued main bearing shells. They do not loose oil pressure.

I wonder if the pressure is being lost at the centre camshaft bearing?

However, I had an engine, Series III, that suffered a broken rocker feed pipe at the cylinder head stud banjo. There was hardly a flicker at the oil pressure gauge. That tells us something about the system's capacity and forgiving nature!

Regards,

Mike A.

[Keith Clements]

Thanks Mike.

Is the running clearance based on the different bearing materials (lead/indium and white metal) or for some other reason?

What clearance should there be on the thrust faces?



I am trying the modern bearing material on all three bearings this time, since we now have a source of two piece rear mains in it. Previously I used separate thrusts on the centre in other engines.

[Keith Clements]

Dear Keith,

Yes, that would be the reason. If Aluminium/tin bearings are to be used the crankshaft journal specification would be:

Standard Size 2.250" - 2.498"
0.010" U/size 2.2400" - 2.2398"
0.020" U/size 2.2300" - 2.2298"
0.030" U/size 2.2200" - 2.2198"
0.040" U/size 2.2100" - 1.2098"

The crankshaft end float should be 0.003" - 0.004". I have found that 0.002" end float is quite suitable.

I have always preferred the same bearing surface for all three main bearings, that is why I have been using the Perkins shells and separate thrusts so successfully.

M. Allfrey

[Keith Clements]

So I will now check, very precisely, the running clearances using my 'Plastiguage'; but will also try to rig up a pressure test rig.

[Keith Andrews]

Do you line bore the block?

[Keith Clements]

KEITH---LINE BORING. No not personally.

Anyway, my reasoning is that, if each of the bearings show correct clearance and the shaft turns when clamped up, there cannot be much wrong. Can it?

Surely if you bore it out, the holes will be bigger and you need fatter and longer (in circumference) bearings.....back to PINCH again
unless you face off the sides of the block before boring.

How do you know you need to line bore?
Get a 2.5 diameter perfectly straight bar?
Use a laser?

PAT-- What simple tests? Please can you construct a step by step guide for the non-mechanics amongst us?

[Keith Andrews]

How do you know you need to line bore?

As the blocks age heat up cool etc they warp...The only way to really tell is specialised equipment in a Engine machine shop
I dont know much about jowetts engines...what I do know is if the mains dont line up exactly with the crank you will run into alsorts of problems...
Im a lazy person...hence I do not like building then have to rebuild...I far prefer driving, reliabliy...
Hence I have never had an engine problem except for things like worn valve guides or component failure.
I take the extra time/money to crack test, skim heads, line bore, balance...even on non race engines.
maybe 9 time out of 10 one gets away without doing this..it is that 1 n 10 that ends up as a headache, and costs more than the intail machining.
In 40yrs of motoring I have never broken down and had to get towed home...except once...I ran out of LPG and had to get towed to a local LPG station to fill up