Engine goes bang bang bang!

[grovesa]

Hi guys,

There something wrong with my engine
When it starts i have a metallic bang noise in time with the tappets moving. It sounds like its coming form the back right hand side of the engine, towards the top (looking at the bulkhead).
You can crank the engine fine, my compression are the same roughly about 120. All my sparks are sparking. I have recently reground/replaced my valves springs etc., on inspection these are all still in placeand nothing seems to have gone awry.
The bang noise is low, rhythmic, and loud (sounds vaguely like hitting the engine with a hammer really hard) not a "tink" kinda noise.

My thoughts were camshaft or cam followers.... how would i tell if it was one of the other?

Any help is much appreciated

Thanks

[mike.perry]

Crankshaft big end bearings

[grovesa]

how can i tell it's that??

and also, by big end do you mean the con rod bearings?

thank you

[bmcecosse]

S0 -you have had the cylinder head off to regrind valves - and immediately there is a bang noise ? Double check the valve gaps. And is the gasket blown where the exhaust manifold connects to the head? BTW - 120 compression is desperately low.......

[grovesa]

I am aware 120 is low but it is a huge improvement on what it was.

It didnt start straight away. I run the car for maybe 100 miles then it started.
The clearences are all fine, checked and double checked.
I replaced all the gaskets when i replaced the head

[mike.perry]

A bang bang bang noise which sounds like it is being hit with a hammer is generally the noise you get when a conrod/crankshaft/big end bearing goes. A noise like a sledgehammer is when the crankshaft breaks

[grovesa]

mike thanks... from your description sadly it does sound like the con rod big end bearings have gone. O dear... how easyand or time consuming are these to do??

thanks for the fast replies

[mike.perry]

Engine out, strip down, crank regrind, new bearings. And since your compressions are low, rebore, new pistons etc. In other words a complete engine rebuild

[bmcecosse]

Crank bearings are more of a rattle - at least at first. Is the oil pressure light on? Is there plenty of oil in the sump? I still think it could be exhaust manifold gasket - check that first.

[Bazzalucas]

Sounds like a good time to upgrade to a 1275 to me!

[grovesa]

i cant upgrade. I want to keep it original!!

My oil light is not on, and there is plenty of oil in the sump.
I check the gasket, no blowing occuring there.

Took the sump off etc. one of my bearing literally crumbled as i removed it. the others seem okay though i will replace the whole set.

How do i know what size a need.. they don't seem to have any markings on them at all.

Thank you

[robedney]

I've actually done this -- seat of the pants -- on the street in San Francisco (many years ago, young and foolish...). I bought the originally specified bearings (assuming the crank looks OK) and some Plastigauge. Fitted the bearings with the Plastigauge in place, torqued them down, then pulled apart to check the clearance was correct. It was, so put it all back together. You need to do this on all four bearings. If the clearance is not correct, the Plastigauge will let you know how far off you are, and therefore what bearings you need (more or less). It's not all that much more work to pull the head and toss in a new set of piston rings while your at it. Plastigauge is cheap. It's not rocket science, but it works.

You do need to poke around a bit and ponder why one bearing out of four failed, check for oiling, maybe replace the oil pump while your in there, etc.

[linearaudio]

The bearing shells will all have the size engraved on the back face; STD or -010 for example. They will all be the same size, but if you are saying that the shell has broken up completely, so you can't see the markings on the back of it, then it's unlikely to have done the crank any favours! It does sound like restricted oil feed if only the end one has gone

[grovesa]

None of the bearings have a size printed on them.

The one that crumble did so as it was paper thin, it was me that crumbled it.

Im not sure about the oil, as the crank and the bearings both had oil on them and between them.
And my crank is neither pitted or marked.

[bmcecosse]

Sounds very much like time to fit a service exchange crankshaft - I very much doubt new bearing shells will fix this....... Bull Motif do the 948 crank for £80 and the 1098 crank for £100 - including all the new bearing shells.......

[grovesa]

Do you know where you can get your hands on an 803 crank? Also, can i not just take multiple measurememts of the journels tn decide the bearing size and to determine how worn the crank really is

Also, should i look further, as its only one set thats gone- the same piston had the worst compression before the new valves, what could have caused the one set to wear?

[bmcecosse]

Ah - didn't realise it was an 803. Usually lack of oil is the cause - but could just be old age.......... Maybe best to pick up a complete 803 engine - there have been a few advertised in the last few days........

[grovesa]

Okay. Thanks for all your help

[robedney]

Wow, that's a very good deal -- and certainty about getting the right shells.

Sounds very much like time to fit a service exchange crankshaft - I very much doubt new bearing shells will fix this....... Bull Motif do the 948 crank for £80 and the 1098 crank for £100 - including all the new bearing shells.......

[bmcecosse]

I should also mention that if this bearing has being 'banging' then the conrod (and the fixing bolts) are likely to be damaged - as well as the crank journal. And so really there are two 'strikes' against just fitting new shells and crossing fingers........