Would a really wide sealing ring.. right up to the edge of the combustion chamber work?.. don't know the coat of a laser cut disc 2mm thick.. but prob less than a pair of heads.. bit of sealing stuff underneath to let it settle flat when torqued up.. or too bodgy? Sent from my SM-G930F using Tapatalk
I’ve been playing about with the engine calculator and see the predicament you’re in. With those big cylinders you need 72cc combustion chambers with a 1.6mm deck to get 8.0:1 CR, but the deck is already 3.5+/- mm with, I assume, no base shims or gaskets. Without properly rebuilding the engine I don’t see any alternative but to clean up the cylinder and head sealing faces and put it back together. What it really needs is cylinders that are the correct length to get 1.6mm deck and new heads with the correct combustion chamber volume to get 8.0:1 CR. It should then go together with standard pushrods and acceptable rocker geometry, decent squish and CR.
So the problem @zedders is facing, is too much deck height and not enough head volume, right? Great info here, btw, I am enjoy following this thread and learning more
Thanks. And if he cuts down the barrels, and buys new heads of the correct volume, the push rod geometry will be all wrong, I assume?
And I'd do that if I thought 70cc+2cc pockets was sensible. If I was getting new heads, which I am, and went 102mm, 68cc+2cc pockets. Every little helps? 2cc less butchery, wider landing area than 104, less CR than I had, a bit more torque on the studs. I'm in two minds New heads with 55cc chambers, slap it back together. 7.7:1 New heads, 68cc chambers, 1.6 deck, new 102 B&Ps. 8:1
No, if the cylinders were the correct length for 1.6mm deck, standard thickness heads should get acceptable valve geometry because the engine width is more or less standard. He has that now but the cylinders are too long and the heads are too thin.
You'd think so but I think what I have might be extra 2mm barrels and heafs decked almost the same so it added up stock or near enough. Correct length barrels and undecked heads should add up to the same thing. Thanks more making me think about that, where it would go wrong is new undecked heads on my existing long barrels, I'd need 2mm longer push rods. It looks like I'm back heading for new heads with 68cc chambers and new 102mm B&Ps. 2321cc but the deck efficiency should help make up for the lost 92cc...maybe. Blurry things!
But that's good news isn't it? Shave the barrels and reuse the pistons, just new heads Like new heads are ever a "just" item.
The engine is installed in the bus where it's staying for this top end malarky. Car park engineering doesn't extend to a new cam. If it was type-1 I might consider it...but yes it would be more ideal and no doubt stronger with smaller chambers, higher CR and a weeny bit of cam. A little extra lift into the bargain would help the low end filling...no..must not think about it!
That’s what I think too but why didn’t Rob just machine the cylinders to the correct length when he built the engine? I would have thought it was easier to machine the cylinders than flycut the heads. I suppose he had to flycut the heads anyway for the 104’s but it’s messed up the deck.
That sounds like a good guess! Would be cheaper on set up time, especially if the heads had to be machined anyway.
I've been chatting with Rob. He was a little more helpful now I have apart and volunteered that he'd calculated he'd need approx 70cc chambers (I hadn't mentioned numbers), an extra 20cc, which he regarded as impossible so he took advantage of the extra length of the cylinders, worked out he could deck for rocker geometry and then carve out the chambers slightly without changing the chamber cc too much. Also that having deck at 2-3mm is worse for detonation than either side of it. Ducks in a row, best of a bad job with the parts I supplied to build a big stock cam engine on the cheap. So now I'm going to cheat and make it possible by spending an amount of cash on parts that's not far off the total original long block cost...could be more. I was short on ready cash at the time but the price of new type-4 big valve heads has halved since IIRC. If the weather picks up I'll see it I can get my calipers on a barrel.
I might slap mine back together with a bit more torque in the meantime. Then I can drive about getting valve job/barrels shortened. Slight spanner - AA don't sell 102mm, just 103, 104 and 105. 103, stick with 68cc, 8.1:1???? Or force them to cut 69 or 70, probably against their better judgement, each additional CC must be harder than the last. 2 days to order in their sale.
By flycutting the heads to compensate for the long cylinders (and retain the standard rocker geometry) he’s compromised the combustion chamber volume and had to cut the head cooling fins. IMHO what he should have done is cut the cylinders for a good deck, increase the head diameter for the 104’s and increase the chamber volume to get 8:1 CR. That’s what you will do when you rebuild it with 102’s but you’ll need about 4cc less chamber volume for the same CR due to the smaller cylinders. Or you could keep the 104’s and fit a better cam so you can up the CR - a cam and followers is probably about the same price as a set of B&Ps. But you’d still have the head sealing problem.
This^ but make sure AA supply the cylinders at the length you want and that all four are the same length.
