Piston Bore condition - need some advice

[dasadrew]

Hi guys,

having now started to clean out the bores I'm faced with a dilemma. The bores all measured up superbly in all directions as being within original works tolerances (just!). I'd planned to reuse the original pistons as I've been cautioned against County pistons and rings.

On three cylinders I'Ve now uncovered some surface pitting. It looks hasty, in reality if you run your finger or fingernail over it you don't actualy feel any height difference, it just feels, err, matt. I presume this has come from water in a standing engine - to be honest, the pitting looks less like corrosion and more like casting porosity!

None of the bores have any other damage or scoring.

So, now I know it's not perfect and Ross Brawn wouldn't give it the nod, I need to work out a practical way forward and need some sentiments from the experienced engine chaps - flying farmer & Co (!).

Am I faced with:

a) waste of time reassembling it. Rings will disintegrate at the first turn of the key. You need a rebore and County oversized

b) if you're willing to accept that it won't last forever, and that it will be a bit of an oil burner, then put it back together and plan for an overhaul in a year or two.

c) nothing to worry too much about. Get a honing tool and run it up and down the bores. Castings were never perfect anyway.

I actually have acquired already 4 x original oversize Triumph Stag pistons and am hoping that another four might turn up sometime in the next couple of years, so would love to go along the b) route unless one of you Gurus says a) is unavoidable.

Cheers for any advice chaps.

Drew

Photos: As usual, anything like this on a photo looks like Mount Everest and is accentuated to the extreme, but the affected areas are shown quite well. The pitted areas are local, but the flash makes the whole bore look messy. Best representation is the last metal coloured photo. Third photo is a "reference" undamaged bore.

DSC00048 (Custom).JPGDSC00050 (Custom).JPGDSC00051 (Custom).JPGDSC00053 (Custom).JPG

[Andrew S]

Hi Drew
When I read your post again I pick on the point that you say The bores all measured up superbly in all directions as being within original works tolerances (just!). From this it cries out rebore and go for the oversize. Remember that good heads and valve gear on a rebuild when you have finished will give you better compression so this will be trying to force oil past these pistons if you put them back, so new oversize would ensure that you wont be doing the job again in time to come.

Just my opinion and whilst it is all apart, its the cheapest option and peace of mind

[Kevin Garrod]

I would be inclined to agree with Andrew, after all the time and effort and meticulous attention to detail that you have made so far, now is surely not the time to cut corners. (very much a non-expert point of view however)

[NickA]

By oversize do you mean you've got half a set of +20' hepolites?

I'd put it back together and wait and see if I could find the other half of your set.

Might be a hunt for rocking horse what not though

[dasadrew]

By oversize do you mean you've got half a set of +20' hepolites?

I'd put it back together and wait and see if I could find the other half of your set.

Might be a hunt for rocking horse what not though

Not sure about Hepolite, but they are original pistons.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=250928312321&ssPageName=ADME:B:EOIBSA:GB:1123

Drew

[V Mad]

Also no expert but have rebuilt a few engines. If the bores were worn there would be a ridge ring around the top, but I can't see much evidence of that.

How did you measure the bores? Use of a piston ring to move down the bore, measure the gap at top middle and bottom, then compare them, is a good method to assess wear. Do you know if the engine ran OK before?

I would run a hone through them and see what they look like then.

Any major machining is always a risk unless you are sure the shop is really good. If the bors are good enough and ran OK before, then a hone, and new rings should be a low risk option. JMTPW.

[NickA]

Not sure about Hepolite, but they are original pistons.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=250928312321&ssPageName=ADME:B:EOIBSA:GB:1123

Drew

Jesus Christ Bananas what a bargain (if you can find another four!)

[stagmuffin]

i think personaly i would be inclined to build what you have because the seeds of doubt are with you concerning the quality of some parts that are available,all the bores are within tolerance .there is no other damage and this is localised and on three pots only. you say no discernable difference with the finger nail test and it doesnt look to be in a hard working area of the pot .whats the risk if your going to put it back in it could give years of service and you would have time to possibly locate all the original parts for the quality of build you want .
steve

[Ian Durrant]

I think i would be inclined to give it a light hone and then reassess.

[dasadrew]

Also no expert but have rebuilt a few engines. If the bores were worn there would be a ridge ring around the top, but I can't see much evidence of that.

How did you measure the bores? Use of a piston ring to move down the bore, measure the gap at top middle and bottom, then compare them, is a good method to assess wear. Do you know if the engine ran OK before?

I would run a hone through them and see what they look like then.

Any major machining is always a risk unless you are sure the shop is really good. If the bors are good enough and ran OK before, then a hone, and new rings should be a low risk option. JMTPW.

Chris, I measured the bores with a bore gauge. The engine ran ok until the PO discovered a blown head gasket. He then left it with antifreeze in the bores for three years or so. When I first inspected the bores, they were oily and shiny. Only after claning off the oil film and running over the surface with a gentle polishing mop did the pitting show. A few bores have a localised "lip" one one side, but it disappears with the polishing mop (a sort of fleece ball) so I suspect the lip was just build up of carbon.

2010 - 04 - 020 (Custom).jpg2010 - 04 - 021 (Custom).jpg

[flying farmer]

Best hone out the bores first, then check your ring gaps. Provided the corroded area can barely be felt (ie no obvious step), and is not big enough for more than one compression ring to be on the same corroded patch you will be ok.
What you don't want is corrosion pitting that goes vertically down the bore as this will lead to blow by and excessive oil consumption.
As there is very little bore wear you can probably get away with quite a vigorous hone which is likely to remove a good deal of the pitting.
Ring gaps are the key to a sucsessful rebuild. Before you hone check the gaps on the unworn bit at the top of the bore. If the gap is bigger than Triumph recomended, it will be worse after the hone. For example if you were to hone out 1 thou will give a three thou bigger ring gap, and I have done this before now. Make sure that you hone evenly down the length of the bore, what you definately don't want is taper from top to bottom.
A tapered bore squeezes the rings in and out on the piston as the piston travels up and down the bore, this wears the thickness of the rings very quickly and leads to the rings breaking, which is why I have given up putting new rings on bores with a significant ridge at the top, even if the ridge is removed you will be lucky to see 20,000miles before the rings and pistons are knackered.
However, if you have a good, straight but slightly oversize bore that leaves you with ring gaps that are at the top end of what is required, you could do what I do, and fit 20 thou oversize rings.
This is a fiddle as each ring has to be filed down to to suit its individual bore, but it does mean that you can set the gaps at the minimum recommended for each ring, which reduces the amount of blow by, and increases power and reduces oil consumption. When I do this I generally leave the old oil control rings in place, if they are in good condition, as the gap on these is not as critical and they are generally the least worn anyway.
I wouldn't worry about the quality of County rings. You never hear of problems when fitted to rebored engines, it is old tapered bores that kill them, and as yours are barely worn they will be ok, though you may get lucky with your original rings
Neil

[dasadrew]

Best hone out the bores first, then check your ring gaps. Provided the corroded area can barely be felt (ie no obvious step), and is not big enough for more than one compression ring to be on the same corroded patch you will be ok.....
Neil

That's a good pointer - I'll measure where the rings come to and check the ring gaps

Thanks for all the ideas guys.

The consensus seems to be, in any case, to hone the bores first and then look see. I've got one of those honing tools that fit in an electric drill, so I guess using slow speed and moving it in a controlled way up and down the bore is the way to go? The stone fingers are quite long, so that should help to avoid making a barrel shaped bore I guess.

There is a good engine shop nearby, but they are really very good, and always want to gold plate everything they do. A simple bore out to oversize costs £60 per cylinder, but I just know them and that won't be the end of the story. Actually, last time I was there they seemed to think that, if I gave them the dimensions of a Stag piston, they would be able to source a suitable one from Mahle or Nüral which could be adapted - for example with gudgeon bushes.

I think if no-one shouts to leave County pistons alone, if I have any doubt after honing then I'll just have to bite the bullet and get a rebore. Although the idea of putting it together and letting it run for a year or two seemed great when I wrote it, I know that I will be kicking myself if that actually happens. The block is out and completely stripped now - it's probably madness to reassemble with the idea of removing it in a year or so.

Drew

[Tonyw]

Hi Drew,

I came across these pistons when I was looking but I have gone with the county option as these were a little expensive, but they may be an option if you want originals.
It would seem that there are two sets one standard size and on one +20

http://www.ebay.co.uk/itm/8-Triumph-Stag-Kolben-85-987mm-V8-OHC-2997ccm-1970-77-/260612219411?pt=DE_Autoteile&hash=item3cadb2e213

http://www.ebay.co.uk/itm/8-Triumph-Stag-Kolben-86-495mm-V8-OHC-2997ccm-1970-77-/260612220645?pt=DE_Autoteile&hash=item3cadb2e6e5

Hope this may be of interest to you.

Cheers
Tony

[dasadrew]

Thanks Tony, if they would have been the right pistons I would have snapped them up. Unfortunately, the 18599 pistons are for the low compression (USA) engine. Thanks anyway - if you find any AE 19983 then you're my friend for life!

Cheers

Drew

[dasadrew]

I'll probably get shot down for asking but, as we recently had a question whether it would be ok to skim just one head, the twin-brother of this question is: Would it be beyond the realms of fantasy to just go to + .020 one ONE side of the engine? (4 cylinders)