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Dummy Engine Build Question


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#16 Cooperman

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Posted 03 June 2015 - 09:58 PM

If you know the thickness of standard ones, you can dummy build using the ones originally fitted and work out the difference. Then you can calculate what the float will be with standard new ones and go from there. However, that relies on some very accurate measuring and for the cost it is best to fit brand new standard ones, then measure the end float with  DTI and correct with +003" or 2 x .003" in order to get it right.

If the crank has had to be faced on the thrust faces it can be necessary to get the +0.030" thrusts and machine them down to suit. I had to do this on an engine which 'picked-up' when being started in really cold weather with the clutch pushed down. The clutch load on the bearings before the oil flowed caused the problem. The size I ended up with was +0.003" on the timing chain end and +0.012" on the clutch side.



#17 skoughi

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Posted 03 June 2015 - 10:11 PM

Ok thanks for that guys, I guess It's one of those cases that " it's the way it's done and that's that!" I would hope that a good parts supplier like minispares would be able to tell you what the thickness of the new ones would be but I accept it's probably best to buy a standard set and check from there.



#18 Cooperman

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Posted 03 June 2015 - 10:17 PM

Quite often you need a standard thickness on one side and +0.003 on the clutch side. They are so inexpensive that it is worth starting out with standard, as they may be fine, then adding thicker ones if required. So, if the float with standards is 0.005", add one side at +003 and you'll have 0.002 float, which is ideal. If it is 0.008" float, add +003 both sides and it will be 0.002".

More than 0.012" and you'll need specially machined +0.030" on one side as IMHO a new-build engine should not have more than 0.005" as an absolute maximum



#19 blacktulip

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Posted 05 June 2015 - 09:39 PM

What is the metric allowance for end float?

#20 blacktulip

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Posted 05 June 2015 - 10:35 PM

I found it, 0.075mm

I measured the deck height to pistons tonight. It's 0.006" from being flush. Can this be taken off or is it too fine for machining?

#21 Spider

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Posted 06 June 2015 - 08:55 AM

0.006" can be machined (milled) from the deck no trouble. I'm not sure if you have checked all pistons / bores but that would be a good idea. Also, when measuring piston to deck heights, be sure to do so along the crank / gudgeon pin line.



#22 Cooperman

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Posted 06 June 2015 - 09:07 AM

With 0.006" it would be best to skim between 0.003" & 0.005".

As 'Moke...' says, make sure you have measured the piston top to deck height for all 4 pistons and that they are at actual TDC when you measure. There will be a small tolerance on each rod, crank journal and piston height which could add or subtract a couple of thou between cylinders.

It sounds as though you are making good progress.



#23 blacktulip

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Posted 06 June 2015 - 09:44 AM

yes, making good progress and i am actually really enjoying it :-)  i only stopped last night because i have work today ha ha.

 

i used my new dial gauge to find TDC in the centre of the crown, then i used a straight edge across the deck and used a feeler gauge in the centre of the crown again. i did this over 1 and 4 pistons. i have 0.008" on 1 and 0.006" on 4. is this a heath robinson way of doing it?

 

i also checked my end float and it is 0.075mm

 

sorry for using metric and imperial measurements all over the place, the dial gauge is metric :lol:



#24 gazza82

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Posted 06 June 2015 - 02:42 PM

Checked my end float and just pushing on the crank nose with my hand it's about .05 mm ... which is about .002" ... according to internet conversion tools anyway.

 

Is that OK? Perfect would be better! :-)

 

 

That's using standard thrust washers as the guy who sold me the engine said it was a good one! I might just double-check the backs of the thrust washers again for raised bits around the lettering but they felt fine when I was lubricating them with a thin coating of Torco MPZ yesterday.

 

 

The problem I have is the main shells I was sent are .020" and my crank is standard!! So it don't drop in ... bit of a delay while I get the replacements .. I used the old shells for now to check the float.



#25 Spider

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Posted 06 June 2015 - 07:00 PM

Checked my end float and just pushing on the crank nose with my hand it's about .05 mm ... which is about .002" ... according to internet conversion tools anyway.

 

 

I think you'll get varying answers around here, but IMO way too tight. Aim for 0.004 to 0.005". These are a case of more is better than less. I'll add though at 0.002" I'd tend to think the thrusts aren't completely seated. If you look on the back of them (the steel side) many have numbers etc stamped in to them and this bruises the material ever so slightly. As a matter of course, I rub them on a diamond lap just to take these and any other high spots off.



#26 carbon

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Posted 06 June 2015 - 07:10 PM

I found it, 0.075mm

I measured the deck height to pistons tonight. It's 0.006" from being flush. Can this be taken off or is it too fine for machining?

6 thou should work fine for a fast road 1275 engine. The important point is that all of the pistons are same to within a thou.

 

The original Cooper S 1275 engines typically had the pistons 35 to 40 thou down the bores. I suspect if you have them this far down then you will see changes in ignition timing at low rpm due to less squish effect, but peak power unlikely to be much different for same CR



#27 gazza82

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Posted 06 June 2015 - 07:12 PM

I might have to agree with Spider. The two on the cap seem to be moving slightly but I haven't got it all tightened down yet due to the wrong bearing shells being supplied. I'll check the thrust bearings again and triple check everything when the new shells are in.

 

 

 

I'm not in a rush with this engine, so I'll check and check .. let's face it it's taken since May 2014 to get this far!! I still need to get the car back home to put it in and that is going to need considerable work first ... including dismantling it to start with!


Edited by gazza82, 06 June 2015 - 09:38 PM.


#28 blacktulip

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Posted 06 June 2015 - 09:45 PM


I found it, 0.075mm

I measured the deck height to pistons tonight. It's 0.006" from being flush. Can this be taken off or is it too fine for machining?

6 thou should work fine for a fast road 1275 engine. The important point is that all of the pistons are same to within a thou.
 
The original Cooper S 1275 engines typically had the pistons 35 to 40 thou down the bores. I suspect if you have them this far down then you will see changes in ignition timing at low rpm due to less squish effect, but peak power unlikely to be much different for same CR
Im not building a 1275 its a 998 +60 I'm doing :-)

#29 Cooperman

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Posted 06 June 2015 - 10:19 PM

IIRC the original BMC Workshop manual quoted the crank endfloat at 0.002" to 0.005".

I've always worked to 0.002" to 0.004" and never had any issues.



#30 blacktulip

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Posted 07 June 2015 - 08:35 PM

do you think my way of measuring the deck height is a bit heath robinson?






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