PeteStupps

I suppose if they were significantly lighter you'd have a lot less reciprocating mass, so the crank wouldn't get as stressed, so you could rev it a bit harder. But probably not much in reality! This is something I was thinking about yesterday. My FWD engine needs a rebore after the exhaust valve breakage, but I don't really know how much so will have to take it to the machinist before buying any pistons, then he'll have to wait for the pistons to arrive before doing anything with the block. Sam I just checked my 'glovebox' workshop manual for the 1296 and 1500 Spitfires and it says: "Piston clearance in bore --------- 0.0009 to 0.0024 at bottom of skirt" [inches] So for a rule of thumb, I'd call it 1 or 2 thou! Interestingly the piston diameters are quoted at top and bottom, and they're slightly smaller at the top - didn't realise that. And the tolerance is tighter at the bottom. For example, a Hepworth grade F piston is 2.8752" to 2.8799" at the top, and 2.8976" to 2.8981" at the bottom Pete Edit: the dimensions in mm are given as follows: Cylinder bore: 73.66 to 73.64 Piston top dia. 73.15 to 73.03 Piston bottom dia. 73.61 t0 73.59

Ah right, I need to pay more attention. Didn't even notice the cross bar!

What am I missing here...? Looks like a standard Mk1 /Mk2 front end doesn't it?

Roger thank you that's a very generous offer, but I wouldn't put you to the trouble. There is a set of NOS Tranco valves with the Herald-type cap fitting on eBay that a bloke on the TSSC site pointed me to (following your mentioning tranco), which I'm going to order. The same ebayer has some NOS ex. valves for the Spit, which I'll probably go for as well just to put my mind at ease (blissful ignorance, no doubt!). The cylinder bore is a bit of mess, as you'd imagine, but I'll get that seen to by the machinist before hunting for oversize pistons. Not sure if it might need more than +10. Thanks Nick and Mike for the warnings about low mileage motors, I have heeded your experience! My engine needs to come out whatever happens, and I need a trip to the machinist for the head anyway, so chucking an unknown non-running block on the car hardly seems like it will save me any stress.

Haha yes! Except I want to get it back on the road quickly, and only have a 12-amp supply in the garage. Maybe I'll just check what 2nd hand Leaf prices are doing though...

I haven't a clue Roger but I know there are listings for "Unleaded" exhaust valves from Paddocks and other suppliers. I wondered if NOS Tranco (or other) would be suitable for unleaded, or if they might burn around the contact patch. Did some idle googling and came up with the following, which may or may not be true: That's from here: https://www.calverst.com/technical-info/valve-guides-materials-and-usage/. Nick I was actually leaning more towards sorting out the existing engine. Seen a photo of the engine bay of the rust bucket, and it looks so weathered it would be a nightmare to get the thing apart. At least mine will come apart easily. Plus I won't have to accommodate and then dispose of another car.

Roger I thought this was some Latin phrase at first! Then cottoned on. Thanks for the suggestion but I was under the impression that exhaust valves needed to be hardened 'unleaded' type these days to match hardened seats, or is that marketing smoke & mirrors? Mick Dolphin has helped me out with a few NLA parts, but his stock is definitely thinning out. Thanks Nick that is some food for thought, made me wonder whether no.4 cyl might have been running a bit too hot, contributing to the failure. There's a water port at the rear of the head, on the top, which is blanked off on the FWD engines (and some others) but which supplies the heater matrix on the Spitfire. I wondered if that contributes to a hot-spot near cylinder 4, but then the same would apply whenever you had the heater valve closed so probably not. The engine itself seemed to be running hotter since the rebuild, as you'd expect with new rings I suppose. Water temp was OK but it developed a habit of vaporising fuel in the pump, which was never a problem before rebuilding. Apart from getting to the root cause I'm in a quandary about how to actually get back on the road. Bloke on facebook is offering a terminally rusty FWD for £200, allegedly with only 50k miles on the engine. If I can get that trailered up I could swap the engine over into mine and see if it'll run. Really I'd want to have a look at the bottom end and bores while it was being transferred over, bit foolish not to. But will that be less cost / time / effort than stripping my knackered but clean and well-lubricated block and getting rebore & new pistons...? This is a question I could spend 3 months pondering.

I'll strip this head anyway and see if there are any tell-tale signs on the other valves.

Thanks Nick. Just checked again and it's actually a Dolly 1300 head, so gives the same 8.5:1 as existing head, but with slightly larger inlet valves. I'll speak to my head man about the options. One of the annoying things is that this is (was) the first car I've owned where I didn't need to top the oil up between changes, because it didn't use any! That didn't last long...

I bought the valves from James Paddock, about 3 years ago now. Obviously I'm very keen to avoid a repeat of this issue, so what do I do with the valves I just bought from Paddocks for the Spitfire?! Am I right to assume all the big retailers get their valves from the same source? Could this have been caused by anything other than a weak valve? I'm not very experienced looking at engines but don't think this looks to have been unduly lean. If that could have had anything to do with it. It was running standard single valve springs, also from Paddocks. Any thoughts gratefully received!

.. and now some pictures of the offending valve, or what's left of it. I don't know if it's possible to tell what caused the failure - presumably not, as the broken end will have been peppered by debris after it broke. I did a side-by-side comparison with an old exhaust valve from the same engine, then against a new inlet valve just to see if it broke somewhere obvious. The old valve is too coked up to see but against the new one you can see the break lines up with the weld (?) line. Are the stem and head welded together? Lastly here is a close-up of the stem end where it broke.

Now some photos of the head. I presume this amount of damage to the combustion chamber means it's scrap... I have a spare 1500 head which doesn't require much work except new valves. The question is where to buy valves that don't fall apart.

I have an update on this sad tale, after getting a few cold hours in the garage. Got the head off, and there is most definitely a hole in the piston! Exhaust valve seat is still securely in position, well battered though. The exhaust valve itself is not entirely present and correct. Looks like the valve head separated from the stem... I've heard of this happening but not seen it before. Is it likely to be anything other than a manufacturing flaw? Here's some pics of the piston and bore:

Thanks Nick, I'll post some pics as and when I get the head off (won't be for a few days) and we'll see what the damage is. If the block is scrap then I'll probably put a dust sheet over the car and ignore it for a year or two! I wondered why they used those push-on valve caps on these instead of the collet type. These are a lot easier to assemble. It's only got the standard soft springs, which probably helped keep things together.

Maybe not totally conclusive but I slackened off the rocker and removed pushrod. The suspect exhaust valve rises up about an inch higher than its neighbours, so either the valve head has shrunk or the seat insert has made a break for freedom.