5 replies to this topic

[alex-95]

I'm just about to start dry building my A+ 1293, I've had the block bored out and bought these pistons made by Nural. http://www.minispare.../P21253-20.aspx

 

I've been doing some measurements of the bore with telescopic gauges and I'm getting 71.12 (2.8") which is the correct size for 1293cc. Measuring the pistons at the largest point below the rings are measuring 71.08 (2.798") giving a difference in diameters of 0.04mm (1.5 thou)

 

i've read a few threads and looks like the gap should be 0.076mm ( 3 Thou), Is this what it should be? As to me 0.02mm clearance "each side of the piston" doesn't seem like enough, Especially when heat is added to the equation?

 

Just for added info the top of the pistons (above the rings) measure 70.4mm (2.77"). Stamped on the top of them is 87-524107-00    + 0.020               71.02 sp  0.03      HVG.

 

If the clearance sizes I've got are not enough whats the best way to resolve it? 

 

 

 

[ACDodd]

Speak to the machinist. I use these at 0.0025 to 0.003" clearance on successful builds. The piston crown actually has the recommended 'spiel' (clearance) of 0.03mm written on the crown. In practice I find that too tight. You might have problems with this being corrected as technically it looks like the machinist has it perfectly set at 0.04mm.

Also double check your measurements, telescopics are a pain to use accurately for bore measurements.

If it needs correcting, honing is the only option. Your block will need to be thoroughly cleaned again after.

Oh and good for you on checking. You are responsible for the assembled clearances in your build, and spotting these is the way towards a successful build.

Ac

Edited by ACDodd, 18 November 2017 - 11:46 PM.

[alex-95]

Speak to the machinist. I use these at 0.0025 to 0.003" clearance on successful builds. The piston crown actually has the recommended 'spiel' (clearance) of 0.03mm written on the crown. In practice I find that too tight. You might have problems with this being corrected as technically it looks like the machinist has it perfectly set at 0.04mm.

Also double check your measurements, telescopics are a pain to use accurately for bore measurements.

If it needs correcting, honing is the only option. Your block will need to be thoroughly cleaned again after.

Oh and good for you on checking. You are responsible for the assembled clearances in your build, and spotting these is the way towards a successful build.

Ac

Thanks Ac, I did wonder if the 0.03 written was what was recommended. 

 

I'll re-measure it tomorrow when I get the measuring equipment again, I might have an internal micrometer that fits the bore so will try that.

 

Ah right, hopefully my measurements are wrong then   I did give the pistons to the machinist so he must have bored the block to the clearance  size on the piston. 

[alex-95]

Speak to the machinist. I use these at 0.0025 to 0.003" clearance on successful builds. The piston crown actually has the recommended 'spiel' (clearance) of 0.03mm written on the crown. In practice I find that too tight. You might have problems with this being corrected as technically it looks like the machinist has it perfectly set at 0.04mm.

Also double check your measurements, telescopics are a pain to use accurately for bore measurements.

If it needs correcting, honing is the only option. Your block will need to be thoroughly cleaned again after.

Oh and good for you on checking. You are responsible for the assembled clearances in your build, and spotting these is the way towards a successful build.

Ac

Re-measured it tonight and got exactly the same for the bore diameters (71.12mm), measuring the pistons I get one at 71.08mm and the other 3 at 71.09mm. So is 0.03 clearance, so the difference in diameters should be 0.076mm ( 3 thou) so pistons being 71.09mm and then the bore should be honed to 71.16mm.

 

 

Another question, I've got new con-rod bolts but not yet got new nuts. minispares from what I can see only sell these http://www.minispare...|Back to search where as the originals have the flange type which they say are no longer available, are these the next best choice or are there better ones with a built in flange?

[Spider]

If - IF - they are a genuine nut, they'll be fine.

 

A little while back I was supplied rod bolts & nuts. One of the nuts stripped the second time it was torqued and that was enough for me. I've binned all those and will now only fit ARP. It's very hard to determine with fasteners if they are genuine and with these, the consequences of something letting go isn't worth the slightest doubt.

[mini13]

just a note here on the heat and expansion of the piston,

 

the pistons are mearured at the bottom of the skirt, and at that point they dont see much heat and wont expand much, at the top of the piston you'll find the diameter much smaller, probably somthing like 20 thou clearance, but it varys due to manufacturer design and material.