73 LB 1641 TP So as per CB Performance website I have a deck height of 1.7 mm and a CR of 7.2:1. So as I understand all is within standard tolerance? Does this mean that I just need to use a paper base gasket between the barrels and crankcase and not any shims? Cheers John Sent from my iPhone using Tapatalk
With a 1.7 mm deck and 7.2:1 CR you don’t need shims or gaskets – just use a little sealant on the cylinder base.
It will feel a bit gutless with such a low compression ratio. Like 'why did I bother with a 1641'? Strangely, if the deck height is too large then the engine may run hot while not delivering its full potential power, the flame from the combustion chamber spreads across the whole piston top while it is still hot near TDC, rather than as the piston starts to fall and the pressure and temperatures begin to drop. Recommendation is usually to aim about 1mm. On my first Vege engine, the heads had been fitted with 1 mm thick steel spacers at the top of the barrels. When I first replaced the pistons and cylinders I left the spacers, but the bus failed my hill climb test where the old engine would still be pushing it at 30mph in 4th gear at the top of a particular hill. So I removed the spacers, the deck height closed in to 1mm , the spacers had made it 2mm, and it completed the hill climb test. ..
The cylinders are too long really – the deck is too big and the CR too low. Assuming the engine has a standard cam I’d try to get the deck down to 1.2mm, CR would then be 7.6:1 or 1mm deck for 7.7:1 CR. As Mike says it will be gutless with a very low CR. You’re not using head gaskets are you?
So, standard cam etc. So if I put a 0.5mm barrel to crankcase shim that should do it then? J Sent from my iPhone using Tapatalk
If you have shims there already giving you the 1.7mm deck then yes smaller shim or no shim at all to give 1mm to 1.5mm deck. As above , closer to 1mm should be more power.
No, the cylinders are too long (deck is too big), you need to remove shims/gaskets to get the deck lower and CR higher. If there are no shims fitted you either need to have the cylinders or heads machined, or live with a very low CR. But it’s not just the low CR that is a problem, with a big deck there will be no squish and the engine efficiency will be poor. You need to have a deck less than 1.6mm for a good squish.
As above, remove any barrel shims you may have already. Adding barrel shims will raise the cylinders, giving more deck height and a lower CR, which is the opposite of what you want. If there are no shims, only way really is to machine the heads out slightly. 55cc in the heads also sounds a bit high for stock kit. From memory, stock T1 heads are 50-52cc. More volume in the heads will also lower the CR. Machining the heads should reduce it.
I have owned a total of 4 pairs of heads 1. VEGE recon engine. 1mm steel spacers gunged invisibly in the crud into the heads. 2. VW branded heads. No spacers 3. A pile of crud containing an engine that was £51 on ebay. The heads on that also had 1mm steel spacers in the crud. 4. EMPI 040 heads on my new engine.. dont know. The thing is that if you have a 1mm spacer or a step at the edge of the cylinder head, your head CC will be about 6cc more than stock. ( its 6cc per mm of piston height) (Stock combustion chamber usually 50 cc) This lowers CR and increases deck height . In this situation, even if you have the pistons reading as basically 0mm deck height to the top of the cylinder, you still have effectively a 1mm deck height above the join. So your 55cc head volume plus 1.7mm deck might be 2.7mm deck height and a 49cc head volume...
You might not like the answers you’ve had here but I doubt if the guys you’re asking elsewhere will disagree. Let us know what you decide to do.
You can take metal off the bottom of the cylinders to reduce their height and in turn the deck height.
Yep, my bad it's been a while, it must have been the cylinder end. If you can get to a machine shop they can skim them. I took 0.5mm off mine using wet and dry sand paper placed on sheet of glass, rotating the cylinder every 10 sands to keep it square. It's only a piece of iron after all. There is a youtube video of a bloke doing it. Not me! Kitchen table job. Use grinding paste to bed each cylinder into the head.
The problem with machining the top is that it closes up the gap between the head and cylinder cooling fins, it’s better to machine the bottom, although it is more difficult. Machining the head is an alternative but that also reduces the gap between the fins – as you discovered. Good luck with lapping 0.5mm off and keeping the cylinders the same length and square – I can hear @lhu1281 spluttering into his tea in disbelief from here.
