1. Can you measure the head volume without the fire ring, I'm expecting it to be 5.9cc less. I'm not exepcting anything different to turn up, but it'll just confirm something against my spreadsheet
2. None of the SCR actually tie up to spec. I was looking at the deck height and how that would need to change to get the CR back "into spec". The DG would need to have 0.66mm deck height (before adding 7cc to account for the flat bowl, see below as I started out thinking the DG and DJ pistons were the sasme shape), the DJ -0.4mm and the MV -0.49mm. None of that makes sense to me as my understanding is
- Conrods are common
- Barrels are common
- Cylinder heads are common, German ones being 23.3cc including the fire ring / HG volume
- Compression height of the pistons must be 3.55mm smaller on the DJ/MV engines If the only adjustment is for stroke
- Piston dish volume for the DJ is 35cc, MV 46cc. I have not done any measurements but have scaled from images and back calculated / estimated that the DG must be around 42cc (any chance someone can measure that, I don't have the gear for the right accuracy)
3. If the compression height was as calculated above the piston to head clearance would be 0.44mm for a DJ (nowhere near enough). I quickly measured one of my DG pistons and got a compression height of 40.27mm, can you check that and also the DJ measurement as I have no DJ pistons
4. I measured one of my used fire rings, it was 0.84mm
5. Gowesty has some updated information at http://www.gowesty.com/tech-article-details.php?id=82. On the MV engine they get a CR of 8.64 which is the same as my 8.61 calc as far as I am concerned. They also have the view that the top of the piston is level with the barrel, but state that the head clearance is 1mm, when the fire ring is only 0.85mm, so there's 0.15mm floating around somewhere, the 1mm clearance can only be true if the deck height is 0.15mm
6. I then measured from the top of compression ring number 1 to the top of the piston and compared that against the witness marks on the barrel. That was interesting as the piston was about 9.05mm and the barrel had about 8.6mm of "unscraped" wall. That's 0.45mm negative deck height which is just about the right value to get the SCR into line with factory spec for all engine configurations. Coincidence ? I don't know, I don't have enough experience of working on WBX to know where the deck height normally is, it does seem to answer the SCR question, but that would leave just 0.4mm of piston to head clearance which is nowhere near enough. 1mm would be just fine, as Gowesty measured
Hopefully this adds to the sum of knowledge. If the deck height assumption is incorrect (probably), then one of the other measurements must need reviewing. There's only 2.4-3.4cc reduction in compressed volume required to get the SCR as factory spec
Here's me measuring the fire ring
and the unscraped part of the barrel