Engine rebuild (take two)

[jpyke]

Here's a few of one of the pistons. The rings have corroded since I cleaned them up literally a day ago. They got damp.

[Ian Durrant]

Neil, I'd be interested in yours (and other folks') views on my bores. I assume that the pics below show a pretty glazed bore? The whole block has been bored to +40 thou, one side is linered, one is not. There is a discernible ridge, but not huge. I've got a set of AE pistons fitted which look like new, apart from a little scoring on the skirts where it is to be expected. The ring grooves look brand new. It may be possible to hone out the bores and fit new rings I guess, but something tells me this is a little optimistic. In the worst case scenario I think I'm looking at a sleeve and rebore? It may sound odd but I'm keen to keep these pistons as I think they're good quality(?) and give a bit of an elevated compression ratio compared to standard.

Its kind of hard to tell in a picture but those pistons don't look like the high compression pistons to me. They look like the county/mk1 pistons to me without the dome.

[jpyke]

The county pump won't make up for wear oil pressure wise. Its not a higher capacity pump it just has a relief valve that doesn't open to a higher pressure. A stag engine in good condition will probably not experience that much higher oil pressure than with an original pump/pressure relief valve. its when its cold and the oil is thick that you really notice it in my experience.

That's a good point. My old pump must have been quite worn I suppose because the hot pressure was dropping to around 10psi and it went back to nearly 20 with the new county pump.

[jpyke]

Its kind of hard to tell in a picture but those pistons don't look like the high compression pistons to me. They look like the county/mk1 pistons to me without the dome.

True, but it's a MK1 engine (luckily I have the original MK1 heads so no loss in compression) and the pistons are definitely AE ones - you can just make it out on the crown in the pic. I don't know how long they've been fitted because it was bored out some time ago. I've got some details in the file I had when I bought the car, but no date. I seem to remember reading on a post here somewhere that the AE pistons were well thought of, but I could have remembered this wrongly.

[Ian Durrant]

True, but it's a MK1 engine (luckily I have the original MK1 heads so no loss in compression) and the pistons are definitely AE ones - you can just make it out on the crown in the pic. I don't know how long they've been fitted because it was bored out some time ago. I've got some details in the file I had when I bought the car, but no date. I seem to remember reading on a post here somewhere that the AE pistons were well thought of, but I could have remembered this wrongly.

If the AE pistons are good (as they look to be), i'd keep them too. If the wear ridge is only slight then you will almost certainly get away with a hone and new rings. it would be wise to get the bores checked for taper and ovality though first I think.

[jpyke]

If the AE pistons are good (as they look to be), i'd keep them too. If the wear ridge is only slight then you will almost certainly get away with a hone and new rings. it would be wise to get the bores checked for taper and ovality though first I think.

That sounds hopeful! From what I can glean looking up their part number on google, these look to be AE Hepolite pistons. I was thinking about taking the crank and flywheel to Oselli Engineering for grinding, tuftriding and balancing so I'll probably take the block too so they can do all of it.

[wilf]

Ask them if they can get rid of any step at the top of the bores, as it can cause issues with new rings etc.

[flying farmer]

As Wilf says the step needs removing.

Piston scuffing like yours is usually a sign or severe overheating at some point and the pistons coming close to seizing. The rear left hand one on my Stags engine was like that, but very little metal was actually missing so it all went back together.

You may notice the wear ridge is bigger on the bank with the liners. I believe the factory fitted liners when they cocked up the initial boring of the block. It saved them scrapping the block but in my experience the liner material is softer than the block so it wears a lot faster.

Hopefully a hone and a new set of rings will give it a new lease of life

Neil

[flying farmer]

Another thought while it is apart, check the back of the block with a steel ruler and feeler gauges, sometimes the block deck warps between the rear stud holes, or the fire ring of the head gasket digs grooves in the rear of the block. Any sign of this wants sorting while the engine is away for machining. About half the blocks I have seen have needed some remedial work here.
Neil

[Matt]

Hi jpike - Thanks for the reply; I thought it was two but just had to check....

[jpyke]

Another thought while it is apart, check the back of the block with a steel ruler and feeler gauges, sometimes the block deck warps between the rear stud holes, or the fire ring of the head gasket digs grooves in the rear of the block. Any sign of this wants sorting while the engine is away for machining. About half the blocks I have seen have needed some remedial work here.
Neil

Thanks Neil, some useful tips there. I have to admit that I was surprised to find one bank linered - I thought that the liners were good for restoring a block to standard bore but didn't think the liner material thickness would be sufficient for it to be overbored - at least not to +40. Seems I thought wrong.

I suffered overheating and head gasket failure on the day I bought the car around 4 years ago. Talk about buying a pup! That's the event that triggered the first top end rebuild. The heads were skimmed and I used the thick Payen gaskets with the extra red sealing lines. Since then its been absolutely fine, but I wouldn't be at all surprised if the decks are not flat. I'll have a look.

I'd also be interested to know how much the head gasket compresses - I say this because I'd like to calculate my compression ratio but also because the heads are getting a bit close as far as maximum skimming is concerned and I want to work out whether I need to start thinking about pockets in the tops of the pistons for the valves.

[flying farmer]

You will never hit the pistons with the valves using standard camshafts, even if the heads are skimmed flush with the valves and using standard thickness gaskets. Standard gaskets compress to about 40 thou, extra thick to about 60 thou.

Biggest problem is getting the inlet manifold to fit in the narrower gap (time with a coarse file fixes this).

Also the cam timing retards as the cam is moving closer to the crank. This can be solved by redrilling the cam sprockets to suit, after checking that the TDC timing marks do actually coincide with TDC.

One engine I rebuilt was 4 degrees out on the crank marks, combine this with skimmed heads and one bank was timed perfectly, the other was one tooth out which had left valve marks etched in the pistons without actually breaking or bending anything.

Neil