16 replies to this topic

[whistler]

Just stripped my 1293, which has done only about 1200 miles and found scuffing on all 4 pistons. The bores seem to be alright but am concerned about the cause.

 

When I initially ran the engine the petrol was contaminated with oil of some description and the oil pressure relief valve (new) was allowing over 100psi of oil pressure.

 

Stripped and cleaned the tank and lines. Fitted old pressure relief valve, down to 65/80 psi oil pressure.

 

Can't think of any other occurrences during its 1200 mile duration.

 

Any thoughts?

 

Vaughan

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[fwdracer]

Has the engine overheated in those 1200 miles?

[ACDodd]

Measure the bore clearance, are they too tight?

Ac

[whistler]

Has the engine overheated in those 1200 miles?

No

[whistler]

Measure the bore clearance, are they too tight?

Ac

I'll put them back in later and let you know.

[whistler]

The only method I could find for 'accurate' feeler gauge measuring of bore/piston gap was to turn piston upside down and insert into bore from top, with feeler gauge set at different thickness. Measuring at 90deg to pin.

 

No.1 piston/bore 0.06mm/0.0024"

 

top ring 0.35mm

second 0.35mm

oil ring 0.40mm

 

Mains were fine but 1 & 2 big ends had shallow grooving, so new bearings and slight buffing.

 

Can't help wondering of bore wash as a result of contaminated fuel is the cause of the scuffing. Hone cross hatching still showing on all bores with just minor marks where scuffing occured.

Pistons are 21253, County brand.

Edited by whistler, 06 April 2016 - 06:48 PM.

[ACDodd]

You need a bore gauge to check for clearance accurately. I advocate larger clearances for these engines. I like 0.0025" on a standard unit and 0.003" on a fast road engine.more like 0.0035 to 0.004" on racing motor. What was your reason for stripping this after only 1,200 miles?

Ac

Edited by ACDodd, 07 April 2016 - 07:07 AM.

[whistler]

You need a bore gauge to check for clearance accurately. I advocate larger clearances for these engines. I like 0.0025" on a standard unit and 0.003" on a fast road engine.more like 0.0035 to 0.004" on racing motor. What was your reason for stripping this after only 1,200 miles?

Ac

I was hearing occasional tinkling noises from the timing cover end. This I've discovered was the timing chain tensioner slipper that had snapped. Just received a duplex setup yesterday so will be putting that on without tensioner.

 

There was a slight oil leak from under the timing cover which I found was from the speedo gear/gearbox end plate and whilst it drove well under power it had an uneveness/jerkyness when travelling on light throttle through traffic. There was also an engine vibration as it went through 2500 to 2800 rpm, so I'm looking to have the crank train balanced.

I had already changed the distributor (65dm4) new electronic coil, rebuilt the hif44 (BDL and new yellow spring), new leads/cap/rotor arm.

Cam is MG Metro.

Head that was fitted was standard 12g940 but had another head rebuilt with 35/31 valves and worked ports which will be fitted on rebuild.

[whistler]

Just had the bore /piston clearance checked by the engineer who bored the block. He checked the bore and it was correct to the 'book' +20 but he said that the pistons had been produced with no clearance.

 

Hope the engineers amongst you can follow that explanation.

 

I queried the fact that he did not bore the block out with the new pistons as the reference measurement but he said that was not the right way. I suppose that if I had to replace the pistons at any time then if they were under size a little then they would be too slack in the bore. That's the way I read it. Does that make sense?

 

Anyway he's going to hone the bores to give a better clearance, probably about 1 to 2 thou.

 

I'd be interested in other engineers methods of determining the exact size to bore. 

[carbon]

What make are the pistons? There's been some pretty scary threads recently, seem to recall another one where pistons were fitted and then the engine was either very stiff or seized because the rings were bottoming out in the ring grooves (rings too deep or ring grooves not deep enough).

 

I would suggest checking all other measurements on that set of pistons very carefully before reassembly.

[whistler]

What make are the pistons? There's been some pretty scary threads recently, seem to recall another one where pistons were fitted and then the engine was either very stiff or seized because the rings were bottoming out in the ring grooves (rings too deep or ring grooves not deep enough).

 

I would suggest checking all other measurements on that set of pistons very carefully before reassembly.

County brand, 21253-20. There's been no noticeable bore wear, honing crosshatching still in evidence and the ring gap is well within tolerance. What is the depth of the ring grooves supposed to be?

[ACDodd]

Hmph, bores must be finished with referece to the actual pistons you are using, just using the book sizes gets you this!!

 

I have a set of Nural (21253's) here at the moment that have 0.001" variation across all four.
 

AC

Edited by ACDodd, 11 April 2016 - 10:28 PM.

[nicklouse]

Was waiting for this.

This is one of the hard things that people still seem to miss.

Measure calculate machine. Repeat as needed.

A good engine is made from matched parts. And engine is made from parts.

[MRA]

I discussed the quality of nural pistons with their german technical director, he was adamant there was no changes from when they went from the UK (AE) to germany (nural) so I showed him the poor machining of the gudgeon pins (as an example) in the nural pistons and I never did get a response about it....... other than we will look in to it, but too be honest I just don't bother using them anymore, when they where made in the UK branded as AE thay where good pistons you could do a lot with, the nural pistons simply lost the plot, the mindset of it is only 70 bhp max so we can cut corners / costs etc  so I am not at all surprised that AC has 4 pistons that are different sizes, I am surprised they where only 0.001" different though as the other measurements I took off of over 40 pistons were shocking to say the least.

 

I now that county are not from europe, and the quality is possibly reflective of the price.

Edited by MRA, 13 April 2016 - 02:08 AM.

[whistler]

Says Made in Taiwan on the box but I believe the bearings are made in Israel.

They were the only +20 pistons that I could source at the time that were of a reasonable price without going into full competition quality.

Should be having my re-honed block back today.