Spitfire & GT6 Forum

I hate to ask this question because for one, I'm afraid of the answer and second, because I'm embarrassed to be asking it.

Anyway, I'm looking for opinions on the condition of this cylinder wall and if you feel it would hold compression.

I have brush honed it twice. The block has been bored +0.020" already. For whatever reason, #2 and #3 cylinders didn't fare well. My guess is that the motor, before I got it, sat with some moisture in it. The motor did run before I tore it down and it did not seem to smoke or otherwise run poorly. I kick myself now, lots, but I did NOT take any compression numbers before tearing it down, so I have no idea what it looked like.

I can feel the pits with the flats of my fingers. The band of pitting is about an inch wide and covers about 25% of the cylinder wall in two different spots (for a total of about 50%) around the cylinder wall at the same height, about 1.5-2" down. I'm not opposed to more honing, but I didn't want to cut too much and be sorry. The pistons are in good shape, but I don't have specific size information, so I cannot measure them (which, for pistons, is a black art, from what I understand). I also have new rings that I will be installing.

The first two pictures show typical examples, while the last one is a shot of either #1 or #4, which are fine.

Thanks everyone,
Chris

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I learn something new every day...especially if I am working on my LBC!
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I'd say it depends what you are aiming for. It might well hold reasonable compresson, it will likely burn a bit of oil, and it will get you down the road.

If you are happy with that considering the work you have done, and will do getting things back together.

OK

But if you are aiming to keep the car or make other improments, I'd be inclined to bite the bullet and do it right with a re-bore.

I see +40 in your future. Would not expect that to hold up over any period of time.
The machinist will be so happy to work on a clean engine.
Sorry

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Wayne
1970 TR6
2000 Jaguar XK8
1949 Triumph Roadster 2000
1978 Spit6 (getting all the good GT6 parts, all poly suspension and Spax shocks)

+1 on the re-bore.

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Martin Keller
Ventura, CA.
Caretaker of a 1974 TR6

Well, I'd only be really upset with myself if I didn't do it and it didn't hold compression. I'd have this beautifully restored car (I hope, anyway) that was fundamentally handicapped.

I have the head at the shop now, so this will join it, I guess. Looking around, it appears that upwards of 0.060" pistons are available. I don't see anywhere in the workshop manual where it tells me the largest bore you can go, but there is a thing in there about boring for cylinder liners, but I hope it won't be necessary for that.

Will the shop just machine out by 0.010" every time until they get the pits all out? Do I need to provide them with the increments of pistons available? I remember reading a post here that some shops won't oversize without the pistons. If I don't know how much needs to be cut, how can I order pistons? Or, am I just ignorant and they have all the tools to tell me how much they need to cut before they do and we go from there. So much to learn...

Thanks,
Chris

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I learn something new every day...especially if I am working on my LBC!
Please visit my blog and website at http://www.roundtailrestoration.com
and my YouTube Channel at https://www.youtube.com/channel/UC8LASST0WuNG0-po4hK0Maw

Chris,
Unless those pits are a lot worse than they look, going to +0.030" pistons will do it. Your machine shop will know for sure. Some shops prefer to get the pistons themselves. Others want you to supply them.
All the best,
Paul

As Paul says. A good machine shop will measure your bores and advise you of the oversize.

Provide those pistons with the block when you take it in for boring.

Have them check the crank while you are at it.

Does anyone ever put new cylinder liners (Sleeves) in the block? It looks like pistons are available for up to +.060"

I had mine sleeved when I built the engine back in '05. Not unhappy with it at all. Uses standard pistons. At the time, the NAPA shop did not have a listing for the pistons and I sourced them from Rimmer, along with the rings. I did wait for the shop to advise me as to overboring or sleeving before I purchased the pistons.

Let the machine shop check it first and tell u how much it has to be taken down. Then order your pistons. When having it bored supply the pistons and the main caps and bolts. Most machinists like to size to the piston spec.and the caps keeps the block twist down some.

+ 1 more on the rebore. Sorry about that. Keep going, and you will have a 1296 soon!

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Matthew
1960 Triumph TR3a
1970 Triumph Spitfire MK 3
2012 Mini Cooper SS Convertible
2018 Jaguar F-Pace
2018 jaguar XE

Each one of those pits has rust in it, rust and jewelers rouge is the same thing. If you don't bore the pits out, you will polish your rings into oblivion in fairly short order. Yes it will hold pretty good compression now, but in 10,000 miles you will have an oil burner with low compression.

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Remember gentlemen, there is a reason why they are not called "The Friendly Suggestions of Physics."

Did it for another make of car when another bore would have been too close to a water jacket.

I see it as a reset.

Just because +60 pistons are available does not mean they will work out.
The factory only offered up to +40, +60 pistons are from the aftermarket.
As said, the cylinder walls are not guaranteed to be thick enough for +60.
These blocks were made by sandcasting, and there is a phenomenon
called 'core shift'. And there is quite a bit of internal structure you cannot
see, water passages, as well as structural support, so wall thickness is far from uniform.
It's not so hard nowdays to have the walls checked, an ultrasonic thickness
tester is well under $150 USD. It is a bit time consuming to do, since it has
to be checked in so many spots, so expect labor costs.

First thing is to check the current bore diameters, you've done a lot of honing.
Frankly, assembling the motor just far enough to test compression is fairly simple
and inexpensive, especially if the old head gasket is in good shape.
As Paul says, it's likely that +30 pistons will do the job, but once it's at the
machinist they can determine what's needed.

As for trapped rust, that is easily removed with a weak acid solution, Phosphoric
acid solution is used by body shops, easily applied with a damp cloth.

But how are the pistons and rings that you took out?
Any electrolytic corrosion to the cylinder walls would also have damaged the pistons.



Edited 1 time(s). Last edit at 2017-01-30 05:58 AM by clshore.

The pistons were OK, though dirty, they cleaned up well. Some of the rings were broken, but only one from the cylinders of concern.

I don't remember what my final bore measurements were, but I don't remember them being out of spec given the machining work already done.

Going to get it to the shop on Tues, I think, and we'll go from there.

Chris

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I learn something new every day...especially if I am working on my LBC!
Please visit my blog and website at http://www.roundtailrestoration.com
and my YouTube Channel at https://www.youtube.com/channel/UC8LASST0WuNG0-po4hK0Maw