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Re building an engine

HenryC
Posts: 144
Joined: Mon Feb 15, 2016 8:31 pm
Location: Surrey / East Hampshire

Re: Re building an engine

Post by HenryC » Thu Oct 10, 2019 9:48 am

Hello,

Regarding the lettering on the pistons. My understanding is: this was usually done to indicate whether the bore and piston were at the higher end of the machining tolerance or the lower end. My Conquest workshop manual gives this information, confirmed by observation of a stripped engine. The block is marked with a corresponding letter to the piston.

If one fitted a a piston that was at the larger diameter tolerance, into a bore that was at the smaller end of the tolerance, then there might not be enough clearance for the piston to move smoothly. This close attention to tolerances was probably only done at the factory during assembly. It may indicate that the bores are standard size but it is possible that a factory (or main dealer) rebore might have followed the same procedure.

Hope this helps.
Cheers, Henry

EL24
Conquest (Drop Head) Coupe DJ252
MGB 8-)

bakergh
Posts: 69
Joined: Wed Feb 24, 2016 1:57 pm
Location: New Forest, Hampshire

Re: Re building an engine

Post by bakergh » Thu Oct 10, 2019 12:06 pm

Hi Chris
Yes it is possible to replace all but the rear main shells with the engine in situ. You would have to remove the front sub frame to gain the necessary access.
I did do this many years ago as quick fix to keep my car useable. In my case the oil pressure had all but disappeared.
If the crankshaft has been ground the shells would have a undersize marking stamped on the back of them.
I think if you are going to keep the car for the foreseeable then it would be better to bite the bullet and take the engine and gearbox out to do the job properly.
By the way if you still have an indicated 20 psi when really hot are you sure the knock your hearing is actually big ends?
There has been much discussion about oil pressure on this forum so it may be a good idea to have a look before you start undoing nuts and bolts.
Graham

Christopher Storey
Posts: 260
Joined: Wed Mar 16, 2016 6:40 pm
Location: Cheshire

Re: Re building an engine

Post by Christopher Storey » Thu Oct 10, 2019 7:44 pm

These engines are not difficult to rebuild, but there are some points which have to be looked at which do not arise on every type of car

1. The bottom end is the Achilles' Heel of the engine. Because the engine is short, the main bearings are narrow, and wear quickly, and as already mentioned the centre cap is weak and has a tendency to fracture if the engine is over-revved and/or if the bolts slacken or stretch

2. The other area which causes problems is corrosion in the water passages in the head(s). The heads can sometimes be repaired by welding and refacing, but sometimes they are so far gone as to be scrap

3. If you reface the heads and/or deck the block, there are strict limits as to how far you can go because of the rather unusual geometry of the valve gear and because if you lower block and/or head too far, by excess skimming, the inlet manifold will no longer fit properly if at all. These problems can be got round sometimes by thicker gaskets, and in an extreme case possibly by sandwich plates, but this is all difficult and expensive


Don't let all this put you off. Get the sump off and conduct the following examination ( others may suggest additional steps or correct my suggestions )

1.Examine the oil pump for scoring of the lid, wear on the gears, foreign object damage etc - I haven't got the clearances to hand but the info is readily available, and in any event oil pumps are cheap enough

2. Examine carefully each main bearing cap ( there are 5 but the larger centre one is the prime candidate for trouble )

3.If there is no sign of fracture , check each bolt for any looseness , and then take each cap off in turn and examine the shells - if the metal has worn through to the backing - shown by bronzing of the shell , and/or if there is scoring , they will need replacement . The bearing size is shown etched on the back of the shells . You can usually get the top shells out by rotating the crank .

4.Examine each main journal very carefully for scoring . You can check the journals for ovality by using a dial gauge. Again, I haven't got the limits offhand but I would expect the permissible limit to be about 3 thou i.e .003 inches . Taper really cannot be measured with the crank in place . On each main, replace everything before proceeding to the next one

5. Repeat this process with each big end looking for the same things - again the existing shells will be sized on the back, and on the big end journals you can measure taper , although ovality again is the more important factor in assessing wear


This will at least give you an idea about the bottom end, and if there is no catastrophic wear or damage, you may get away with in situ replacement of the shells

Good luck with it

chrismpw
Posts: 3
Joined: Tue Oct 08, 2019 11:22 pm
Location: cricklade UK

Re: Re building an engine

Post by chrismpw » Fri Oct 11, 2019 12:10 am

Chris, thank you. This insight ties beautifully with mutterings given by my elderly neighbour (retired engineer from the motor industry) and other comments I've heard. It helps develop a picture of what's going on and what can be done.

Strangely, I went to a very highly regarded classic/vintage engineering company today to seek their opinion and they heard nothing at all wrong with the engine. My neighbour, and friend, who has nothing to gain from this, is adamant. I also hear it myself. Perhaps the oil wasn't warm enough when I got to the engineers.

I also observed that my gauge reports 18psi of my warm 20psi just by turning the ignition on. I suspect the pressure gauge/sender might be less than honest too!

So two days ago I bought what I thought was my life -long obsession and dream car - today I realise i can't play with it until I pay around 33% more and wait patiently. To cap that today my wife was made redundant by the abandonment of the Dyson car project.

Cars eh? Good job she's a looker. (The car , or the wife....?)

Still, as the universe careers towards its inevitable heat death as entropy has it's way, little is of real consequence.

Sydsmith
Posts: 814
Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: Re building an engine

Post by Sydsmith » Fri Oct 11, 2019 9:37 am

Chris, don't get too carried away with that oil pressure. My SP has about 20lb when hot, the engine is high mileage and is much noisier than a modern engine or my V8 250 under the bonnet, I can't tell if any of the noises are bottom end but as long as the oil pressure holds around 20llb when hot, I don't worry.

My V8 250 has a Russ Carpenter rebuilt engine which is a dream, 5,000 miles no oil leaks, no odd noises and 40lb oil pressure when hot.

In your position I would first get the oil changed, change the oil filter, then try again.

I am not saying ignore the signs, but I would do the above and run the car gently for a few more miles and see if the problem gets worse before I would attack the engine.

They are very robust engines and though the bottom end is the weak point, they have been known to run with the bearing shells completely worn through, there is a thread about this on the old forum.

Hope you solve your problem and can enjoy your very fine car. Syd

Warsash 2
Posts: 302
Joined: Sun Feb 28, 2016 1:53 pm

Re: Re building an engine

Post by Warsash 2 » Fri Oct 11, 2019 11:14 am

Hi Henry
Thanks for this. In my simple mind the A would be the smallest and B the next size up so that is very useful information.
The pistons and the block are marked and as there is no oversize bore marks I am assuming the engine has not been rebored. Only taken to bits and put together many times by 'enthusiastic' students so all the bolts will need replacement.
Thanks again

Colin

HenryC
Posts: 144
Joined: Mon Feb 15, 2016 8:31 pm
Location: Surrey / East Hampshire

Re: Re building an engine

Post by HenryC » Fri Oct 11, 2019 5:35 pm

Hi Colin,

The following text is copied from the Conquest service manual. The engine has cylinder liners of nominal 3.000" bore.

After insertion the liners are fine bored and honed to the 'standard' bore within stated limits.
Finished bores of the higher machining limit are stamped with an 'A' on the offside of the face adjacent the cylinder and are fitted with similarly marked pistons; pistons and cylinders marked 'B' tend toward the lower machining limit.

If Grade Symbol is 'A' then Cylinder Bore is between 3.0003 ins and 3.000 ins

If Grade Symbol is 'B' then Cylinder Bore is between 3.000 ins and 2.9997 ins

Note. After reboring or relining, the bore grade 'A' or 'B' on the cylinder face is longer applicable.


So, if you have matching grade letters on the pistons and respective cylinders, it is very likely to the original standard bore.

Hope this is useful.
Cheers, Henry

EL24
Conquest (Drop Head) Coupe DJ252
MGB 8-)

Warsash 2
Posts: 302
Joined: Sun Feb 28, 2016 1:53 pm

Re: Re building an engine

Post by Warsash 2 » Sat Oct 12, 2019 1:29 pm

Hi HENRY
Thanks for this I hope to get back working on the engine in a few weeks so I will check the bores then
Regards

Colin

tjt77
Posts: 501
Joined: Wed Mar 23, 2016 7:01 pm

Re: Re building an engine

Post by tjt77 » Wed Oct 16, 2019 12:01 am

whilst you can drop the sump (after removing radiator and engine mounts to get the lower part of engine high enough to be accessible to move the sump) and do a thorough inspection, the likelihood of a scored/oval crankshaft is strong.. your low oil pressure is likely the result of worn main bearings.. so pull the center main bearing cap first..
IF the engine still has its original Vandrvell bearings ( they have a 'VP' insignia stamped upon them.. and the copper backing will evident on the lower center main shell) you MAY get away with replacing the steel bearings ..if you have aftermarket 'bi metal' (reticular tin alloy) bearings..the crank will almost certainly be worn and scored, thereby requiring complete teardown and overhaul..

better accessibilty can be gained by removing the front subframe as an assembly.. it generally saves time to do so and will give far better access and vision when you do drop the sump..

tjt77
Posts: 501
Joined: Wed Mar 23, 2016 7:01 pm

Re: Re building an engine

Post by tjt77 » Wed Oct 16, 2019 9:01 pm

Incidentally.. I purchased a late V8250 engine from fellow in the west country a couple of years back ..described as having 'been fully rebuilt by a technical college' it came from a manual overdrive car ( hence no set up for the auto box kick down) and I also purchased all the manual conversion bits including the gearbox from same fellow (Mark) too ..and he received some other parts I had sent to him, including a late manual/overdrive box for a Mk2 jag.. things got rather busy for me since I have not followed up for some time..but he was going to store it until shipping could be arranged.. Hope you did not purchase 'my' engine..

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