First of all thank you all for your patience and tenacity, things have moved on.....honestly. So now the heights are as follows: No1, 0.013" No2, 0.011" No3, 0.005" No4, 0.008" I reckon if I can get Nos 3 & 4 somewhere on the high side of 0.010" I'm good to go? I intend to take the material off the tops of the relevant pistons to make things equalish. I'm aiming for 0.060"or 1.5mm clearance, so if the deck height is 0.010"+, I'll need 4 x 0.050" shims. does this sound like a plan? Thanks for reading this.
You haven’t mentioned if the dimensions are different from one end of the pin to the other – did you find out what was wrong before? I wouldn’t remove material from the pistons until I found out why there is a difference between cylinders, especially 3 & 4. The compression height (pin to crown) should be the same on all pistons, the con rod lengths should all be the same, the cylinder lengths should also all be the same and the tops of the cylinders should be flat to each other or the heads will leak. There will be a machining tolerance on each component but with selective assembly of rods and pistons you should be able to get the deck height very near equal each side of the engine - #1 should be the same as #2 and #3 should be the same as #4, any difference between sides can be corrected by cylinder base shims, not by machining the pistons. The calculation is correct but what CR will that give you?
^this. Shim the barrels if you must. Do you really want to machine the pistons? Fit a 123 distributor, and it’ll accommodate any small differences in compression
The spark balancing, if you turn it on. It’s watching the crank go round. If it speeds up a bit due to greater compression on one cylinder, or several, it retards (I think) the ignition on whatever cylinder to slightly reduce the power and even things out. They’re very clever.
I can't think in thou. 3 thou is 0.075mm. The thinnest ultra lite silver packet rizla is 0.01mm I think the head would conform if you just bolted it up? But I also (probably wrongly) calculate the difference between your lowest and highest is equivalent to approx. 1.5cc and that surprised me.
Its about 6cc per mm from memory. You wont tell the difference between the sides, provided the tops of the cylinders make a straight line from front to back on the engine, so the heads seal OK. You are maybe sacrificing more by aiming at 0.050 instead of 0.040.
.. If you've got the CR room I'd head for 0.04 for a bit tighter deck... (Or 1mm shim in new to get you 1.14 to 1.35mm deck) Sent from my SM-G930F using Tapatalk
Thanks for the response. I agree 0.04" would be more efficient, however, is there still adequate clearance on a standard engine?
Don't think the 0.04" is a problem and you'll have another 0.01" min on top of that I think the safe recommended range is 1mm to 1.5mm total so 0.04 to 0.06" total deck height Aiming for a total deck height of 0.04" is quite common stateside think 0.03'" is where people start having problems Sent from my SM-G930F using Tapatalk
I set my T1 202cc deck to 1mm...then bottled out just before I started it up all installed. Took it back out, fitted 0.5mm shims and relaxed...until I'd run the cam in. Then I took it straight out AGAIN, stripped one side just to peep at the cam and followers...ok...put it pack together again. Nuts!
I agree, don't set it too tight - I had one in the office that was fine until it was properly warmed up but then a piston started to just kiss a head. Hyper pistons might not be so bad for expansion. Was a tad under 1mm, 0.9mm? Type1 engine but same difference?
I settled on 1.3 to 1.4mm total, using a 0.01" (0.25mm) shim, no shims would have taken CR to over 9, got a bit of extra duration on the cam so dynamic CR pulled it down a bit too. Hopefully actually get to drive the thing soon and find out what its like! Sent from my SM-G930F using Tapatalk
A difference of 0.003” from one end of the pin to the other is more than I would be happy with and means the piston is at an angle in the cylinder, but nobody else seems to be concerned, so… And 0.008” difference in deck from #1 to #3 is more than I would expect, even with used parts. You get a good squish between 0.040” and 0.060” deck - you gain very little making it too tight but lose a lot if the deck is too big. There is not much point going tighter than 0.040”, although you could without any risk of the bits that move getting too intimate with the bits that don’t move. If this is a bus engine with a standard bus cam, I’d aim for 8.0:1 static CR and if it went a point or two over, I wouldn’t worry. It’s a case of diminishing returns and you gain little by running a standard bus cam with a high CR. And many will say use a static CR of 7.5:1 or even lower.
I built an engine which had 2mm deck height including step in cylinder head. Drove it around for a couple of weeks. Hated it. Took out the steel spacer put in there by some rebuilder, seems like it was common to machine off the step and fit a steel spacer in the head. I have owned 6 heads, two new, with solid ali head, four with steel spacers.. Vege ? Pulled spacers, back to 1mm and happy .. engine was just a bit dead before. There was a hill where if you left the roundabout at the bottom in 20, you could get it up to 50 going up it in 4th before it got too steep and it would just about hold over 30mph in 4th at the top. With the spacers it needed 3rd gear and was doing about 30 mph. This section here going southeast towards the top of Portsdown Hill.
2mm is a problem, not squishy enough but too small for a burn. I did same as mike to a 1600 I had and it pepped it right up.
