But you wrote 'I will of course take volume measurements with and without the shim.' Take measurements with no shims anywhere. Choose the CR you want to aim for and either calculate or use one of the on line calculators to determine what size shim is required at the bottom of the barrel. I use this one http://johnmaherracing.com/calculators/engine-calculator/ Alternatively aim for a deck height and decide if the resulting CR is acceptable. If there is a 1.8mm step in the head the pistons will be above the top of the barrels to have an actual deck between 1.0mm and 1.6mm.
That was because, as was explained to me here, there was a possibility that the step in the head could have been machined out and replaced with a spacer ring. Removing it would therefore reduce the volume. I'd still be curious about what the difference in volume would be with and without the spacer. As it happens the heads still have the step and are unmolested. As they should be, because they were described as new when I bought the remanufactured engine. They are a couple of the parts I'm reusing from the old engine. So the issue is moot as I don't need to do anything to the heads. I'm slowly getting there but it's still all just theory to me. It won't fully sink in until I've completed each part of the build. And any mistakes made will have to be corrected and learned from before moving on to the next stage. This is as much about learning a new skill as it is about getting a new engine. I've just trial-fitted one piston (no spacers) and the deck height is roughly 1.9mm. I was expecting the piston to be proud of the barrel and therefore need a spacer. So, as has already been pointed out to me, it looks like I'm going to have to remove some material from the barrels. I'm not keen on dismantling the case again for machining. Obviously, I'll need to take more accurate measurements of deck height but, once I have measured the head volume, I should have enough info to go back to the machine shop with how much to take off the barrels. And yes, those calculators are a godsend. If I've got any of that wrong, feel free to shout at me. It's all part of the learning process.
I think some companies just used to fly-cut the step off, then fit a spacer, for ease of head refurb. My old Vege had it done.
Yes, Autolinea. https://www.heritagepartscentre.com/uk/043101025al-crankcase-1300cc-1600cc-aluminium.html Sent from my SM-J600FN using Tapatalk
That would no doubt explain it, but you wonder why they'd do that, turning down pistons isn't such a straightforward job and you should have to do this for a stock build IMO. I would be perhaps more inclined to have the case decked than shorten the barrels. Shortening the barrels is sort of making yourself trouble for the future in that if you needed to replace the barrels you'd be back to machining them. Do the case and it's done. IIRC stock would measure 200mm across the case measured where the barrels sit when it's bolted up i.e. 100mm a side if you can measure it.
I wouldn't machine 2mm off a case, especially not a new one that cost £1,000. Measure the cylinders, they might be shorter than they should be and machine the step out of the heads if necessary.
1 mm of spacer is worth about 6cc, from memory. It's also interesting what brand of cylinders you are using.. AA 1641 cylinders seemed to be in the right ball park with new EMPI heads with steps in them on an Autolinea case . No shimming or machining needed. That's what the VW Engine company used to build a mostly-new-parts engine a couple of years back.
OK, so I like Zed's idea to deck the case. I like 77 Westy's idea to machine the barrels and heads. I also like mikejames's idea to use 1640 barrels. But which is best? There's only one way to find out..... Fiiiiight
You'd only need 0..5mm off the case, no idea where the 2mm came from. And get rid of the step. Good suggestion to check your barrel length is stock though. Just my opinion, there are many ways and it does come down to how much you trust your chosen machinist. Not much faith? Don't let them have the case.
^this. If you don't trust them, don't let them anywhere near your spanky new case. If you do trust them, get them to bore it out for 1776
All very good advice, thanks. So If I understand correctly. Removing .5mm from the case reduces the deck height from say 1.7mm to 1.2mm. Then removing the step in the heads removes approx another 1.6mm and moves the head down decreasing the combustion area but retaining a 1.2 mm space between the piston and the valves. If the head CC is around 45-50cc that would get me in the ballpark of 8-8.5CR? Not measured CC yet but I have some acrylic discs on order to do this. I can turn one of them down to fit inside the step. Hopefully, that should tell me the cc as if the step had been removed. Using a calculator... Bore = 85.5 Stroke = 69 Deck = 1.2 CC unknown but say = 46 CR = 8.5 Aircooled machining in Bristol may be a problem though. I've used Ashley Engines in the past and they have been very good but they are not aircooled specialists. Also, when I picked up my crank from them recently it transpired that they had sent it out for someone else to do. I'll have to have a conversation with them to assess how confident they are. Regarding the idea of switching to 1640, I don't understand how that works. Are the barrels shorter than my 1600 set? I have a set of Mahles which are 136.5mm in height. But barrel height does not seem to be listed anywhere. I'm guessing that machine work is going to be around the £300-350 mark and about the same price as a set of 1640 pistons. So not a lot in it cost wise.
If your going to have the case and heads machined then it would be worth having them opened up to take bigger barrels 1776 cc will give you a noticeable and useable increase in power over stock
You're on it. They key thing is do all the calculating of possibilities before you spend any money on anything. Lots of ways to skin this cat. My personal direction is always I want to get something from spending money, so if for example the case needed machining anyway, calculate for building a 1776 - you could find it's cheaper to build than a stocker. Not saying you would, just it pays to see what you can do with the parts and resources you have. Quite often I read of people who bought one part or another and go to lengths to make it work when it might have been better to start again and use a more suitable part. Sorry for being so vague. I remade a 2020cc engine with a 76mm crank. I calculated I could make it all work out with a tight deck and the right CR for the cam if I used B pistons and slightly longer conrods. A previous attempt by someone else with the same case crank and B pistons, stock rods, about 4mm deck and 7:1 CR with a sporty cam. It was of course total pants. Really, it just wanted longer rods. One simple thing probably added 80HP. lol. Yep, was that bad. I have never heard of having to machine stock barrels so that's interesting and maybe there's something odd going on there or maybe it's the case. The engine above was built on the same case you have. It didn't need decking, I worked out a different way that worked. If I'd been building a 1600 maybe I'd have had the same problem, but my reaction would have been "what could I build if I'm in for case machining anyway?".
There's a "death zone" with deck height from about 1.7mm to about 3mm. An engine with 3.5mm deck will run fine too without destroying itself if you get the rest right (CR mainly)... mine does, it's big cc, lazy and hopefully won't break because I deliberately did not try to eek out every possible HP. We're not going racing.
I’ve heard a couple of reports of replacement barrels being longer than stock over the last couple of years. I wonder how common it’s become When I last spoke to Rob before he sent me my short block he said he’d resurfaced the case where the barrels sit and had to take a lot off due to porosity in the casting. He couldn’t remember how much he took off though It’s sure going to be interesting when it comes to fitting the barrels and pistons to measure deck height.
All a 1641 does is use thinner cylinder walls to gain 73cc displacement at 69mm stroke or about 4.6%. But its intended as a no machining option often used because "its bigger, surely cant do any harm, doesn't cost a lot" I am pretty sure that replacing a 34PICT-3 with dual carbs (or even a progressive) and improving the exhaust makes more of a difference than going from 1568 to 1641 cc swept volume. Now I am going to have to go and measure an AA cylinder and a Mahle to see if theres any difference in the height from the case to the top...
I don't think that applies to stock size but go over that and yes, they do it on purpose because it works out for certain common builds in the same way a 78.8mm crank does (IIRC). Some builds just work with little effort, some are a reet PITA. It pays to do your homework. If I'd been building my current motor with the pile of random parts I threw at Rob, I'd have made changes during the process, I might have even completely about faced and made a stroker with smaller pistons but generally the bigger you make 'em the more problems there are so it would still have been difficult as Mr Westy will agree I'm sure. You definitely do not want "gotta be done by" date..
The other side of that is a 1776 with stock everything including carb and exhaust is cheaper all in and better. Assuming your machining costs less than twin carbs and a new exhaust. People go 1641 because they don't want to split the case and machine, but if it's a new build it's a different circumstance.
Note to OP. Don't trust that case just because it's new. The oil pickup would not fit on mine and on another (super race version) one oilway had not been drilled right through (a major one from the oil pump!). Likely the oilways have swarf in them too. You can't just bolt these things together, particularly these days where almost all the parts will be aftermarket. Where's this leading? To a machine shop with the experience to check these things, pull out the oil things, wash the case thoroughly... Back to the one in my photo I remade. When I split it, there was a long twisty piece of swarf sticking out of a cam bearing oil hole and the thing had obviously not been prepped properly.
