So as a newbie aircooled engine builder, deck height has me confused. Can someone explain what the difference is between shimming between case and jugs and/or jugs and head? Seems both methods do the same thing.
Both would do the same but if you shim the head you have combustion pressure trying to escape, shim the barrels and it's just a bit of oil splash. EDIT: Zed types quicker than me but doesn't use so many words
^what they said. Usually shim at the bottom to shift the barrel up a smidge if it needs it. It will lower compression.
Always shim the barrels, no gaskets, shims or spacers at the head. Deck to suit the CR the cam needs and ideally between 1.0mm and 1.6mm to maximise squish.
OK thanks. So do the calculations and shim from the bottom. Is shimming the top a no no then? When I dismantled the old remanufactured Remtec engine (case being scrapped) there was a 1mm shim under the head. Also, the main bearing had been shimmed and oversized oil valves fitted. Did I buy a lemon?
I don't fit anything between the head and the barrel, just one spacer at the bottom. Re-manufacturers do what they do because it's quick and easy, not because it's correct.
Remtec in particular had a reputation for machining instead of measuring. If you have the pulley, measure the oil thrower spiral diameter. IIRC stock is 49.75mm but remtec also machined the case here and supplied 50.25mm spiral pulleys which is obviously an interference fit on any other case and if you have a remtec case like that it's that over-sized pully or leaks.
The steel spacer ring on the head is a remanufacturer trick to make sure that a head that may have been damaged by exhaust leaks will seal again. They replace e.g. 1mm of aluminium machined off with the same height of the spacer ring. Be aware that if you remove the ring there is no "step" in the head , and so effectively the piston comes 1mm closer to the head.. so you need to bear this in mind when calculating the total deck height. With the shim it would be Ok for the piston to come to almost precisely the top of the cylinder as there will still be a 1mm gap. Remove the ring and now you need to add a spacer below the cylinder to keep at least 1mm between the piston and the metal of the cylinder head. On my steel head ring equipped VEGE engine they had also added spacers below the cylinders to make the clearance even bigger .. as fundamentally.. low compression ratio engines dont make as much power so dont get so hot so dont get returned by customers so often.. Head spacer ring and base spacer ring, 1mm thick , stock barrel size. If the heads aren't cracked and the valves seal or can be reground to seal and the valve guides aren't too loose the heads are good to go.
What he said ^^^ You are aiming for 50 thou or a tad over 1mm Lots of good info here and his other sites. One example https://www.aircooled.net/2275cc-engine-combo/ Too many shims and the deck heigh is too great. Get it wrong and the motor is a pig. It is a balance between, deck heigh, head chamber, shims, and decking the case A good engine builder its easy. A novice and a bad engine builder - its a black art. A bit like a street fighter and a surgeon Both stick blades in you. One knows how to make you survive.
So for example, if my starting deck height was say 1.9mm and I was aiming for a final height of .050"/1.3mm I would use a .6mm shim, and then adjust the head volume as required. Am I correct that the stock CR for a 1600dp is 7.5:1.? I won't be doing any additional head work as they are in pretty good order with no leaks. Those CC kits seem ridiculously pricey so I'm going to have a go at mocking up my own CC measurement rig using a 50ml burette I picked up from work and a perspex disc cut to size. I'm assuming the disc has to sit snugly on top of the head shim? I will of course take volume measurements with and without the shim.
Yes to everything! I used a Perspex disc and antifreeze (it's coloured ) to measure the head volume, valves and plug in. From memory (may be wrong) stock volume is about 40-42ccs on a 1600 head. You could push the boat out and go a bit higher than 7.5:1. <edit) 50-52ccs
Don't use a head shim and definitely don't try to measure combustion chamber volume from a shim. Measure everything then calculate what shim you need at the bottom of the cylinder. If the starting deck height was 1.9mm with no shims and you are aiming for a final height of .050"/1.3mm you'd need to remove 0.6mm to reduce the deck. Adding shims increases the deck height. The standard CR is extremely low, even for a standard cam, I'd aim for 8:1 or a bit more but it might be a compromise between deck height and chamber volume.
There was a possibility that Remtec had removed 1mm from the step as a repair. Turns out they just shimmed from the top rather than the bottom and the step is a good 1.8mm. That makes things a lot simpler. Sent from my SM-J600FN using Tapatalk
I'm still lost. If remtec put shims in, take them out. If there is still a step left all well and good but shims at head end are not good, a known failure point. Then do your calculations.
No. As said, you need to remove .6mm from somewhere. * Shave the top of the barrels. * Deck the case I “think” heads should be around 55cc for best volume.
Precisely my point. No need for the head shim. It was just the cheap method used by Remtec. I, on the other hand, will be following the advice from this list. Do it right, do it once. Sent from my SM-J600FN using Tapatalk
Yes of course. Got that the wrong way round. That was just as an example. Will do some proper measurments at the weekend. Sent from my SM-J600FN using Tapatalk
