50 replies to this topic

[Pigeonto]

Unless the pistons are 2.839? You would know alot more about sizes i.e. whether the piston should be 2.840 less the toleramce or if it is full size and the bore taken out to as you say,nearer 2.841.The pistons were obviously sent in with the block.This fella has done the job all his life and I just left it to him,he does all the work for a local classic car specialist too.I insisted on the very minimum tolerance on the big ends and had a bit of an indifference with him after he ground the crank to its biggest tolerance for 'safety' as that's what they do to make sure nothing locks.I made him re-do it,sadly to -020" and give me 3/4 of a thou running clearance...he actually did 1 thou clearance but I was happy enough and that side has been lovely,,BUT,, I wonder did he at the same time think I wanted it tight on the piston clearance too?? which I didnt.Sorry AC, pistons were CAHT....something minispares but 99.99% sure they are AE with the solid skirts not those flappy slipper type

Edited by Pigeonto, 12 February 2013 - 08:12 AM.

[ACDodd]

Are the pistons galled on the skirts? Mini spares standard pistons would be a nominal 2.8405" depending on cam choice I bore mine from a minimum of 2.841" to 2.8425" depending on the cam and application. People do not realize fitting a warmer cam means more piston to bore clearance. I never use the stock clearance, too many engines stripped to find galled skirts. Typical fast road build on the Mega pistons would be 0.003" clearance, rising to 0.004" for a competition build. 286 cam are aim for 0.0035". The most powerful A-series I ever built ran 0.004 to 0.0045".

AC

Edited by ACDodd, 12 February 2013 - 05:43 PM.

[Pigeonto]

Still all together AC.I roughed up the bores like that...well I reasoned I coulnt make it any worse,and now have the uprated head on which I wanted to try .I have to say that it doesnt smoke at the moment and I cant smell any oil.Am running it so weak it keeps stumbling and I can breathe the exhaust!!
You certainly have me concerned now about the clearance.I am happy tho that the bores are not atall oval or worn so if it is undersize then fair chance it will hone out to size.Doesnt that mean the piston can tip and the rings then arent 90 deg to the cylinder wall??

[ACDodd]

Read the last sentance, re loss of seal.

AC

[Pigeonto]

OK. Well I will just run it a short while and see if this is the head I want on it and then strip and measure everything.Good time to change the baulk ring that seems to have given up.I remember they were definately AE pistons, would have to get Simon to tell me what they supplied,whether that tells you the likely size.I have been told the pistons expand more front and rear ? if that is right, maybe it would explain the rubbing in that area of the cylinder walls

[ACDodd]

The Mega pistons are branded AE.

AC

[Pigeonto]

Oh dear. Looking rather ominous then.I may well be taking block and pistons back to the machine shop for measuring/bashing him over the head with. Dissapointing after all the time and trouble.This was supposed to outlast me

[ACDodd]

Well you have to be carefully here as the block could well have measured the right size when it was machined in a warm workshop. If you measured a cold block it could easily be more than 0.0005" smaller. My suggestion is remove the engine strip and then check when cold. Hone the cylinders to give the right clearance when cold. Fit new rings. Also check pistons if they are damaged replace these as well.
If it helps I add 0.0007" to my finished bore sizes in summer to allow for winter cold starts.

There is a lot more to bore sizing than just the correct diameter when machined.


Ac

Edited by ACDodd, 13 February 2013 - 04:29 PM.

[Pigeonto]

So I see.Yup, the machinist and I were discussing this sort of thing when we had the row about my crankpin dia's and said he would want it in the w/shop where the micrometer etc are so they were all at the same temp. Update on progress:- Engine back together with new head on,did 50 miles and smoking a lot less prob due to roughing up the cylinder walls but tested at 25% leakage on all It's answered my question about the head and I'm definately using it, good improvement so I'll now strip and do as necessary,before I get a step on cylinder wall.Thankyou for responses.Going to run it in stupidly weak,then a trip down to ML for setting up.

[Cooperman]

Good luck with getting it all sorted.
It's surprising how many people don't realise the difference in thermal expansion between aluminium alloy and cast iron.

[mk3 Cooper S]

.Going to run it in stupidly weak,then a trip down to ML for setting up.

Beware of running stupidly weak too

[ACDodd]

Stupidly weak is perfectly fine at part throttle loads, and actually beneficial too.

AC

[mk3 Cooper S]

No it isn't for max cylinder temps, I have melted enough pistons on the dyno !!!

[ACDodd]

Not at Part throttle!! Full load lean is death to an engine.

AC

[mk3 Cooper S]

Depends how lean is stupid lean.
If it is stupid lean you will be at full load before you know it to make any sort of progression