Any one know the bore diameter for an M28/43 (85-86 auto 5L) ?

I'm playing around with the knock sensor settings on my Mega Squirt and I'm trying to work out the resonance frequency of the bores......

 

the pistons were exactly 100m... the gap was only a few thou. to the bore.

 

Let me know how you do that calc. In my wayback history, I started by using a spectrum analyzer connected to the piezo sensors to try to identify the signature of knocking, while working to isolate that from piston slap and other normal combustion noises. Since the knocking occured only when piston was at or near TDC, the noises I was looking for started out in the head. Except for RPM multiples, which obviously move around, there wasn't a specific frequency signature that I could identify with knocking. In the end there was some filtering, but my controller was looking at peak amplitude spikes vs. a moving average as best indicator.

I'm sure the science has improved. And while I worked with rocket scientists a lot, I didn't have one on call to verify my assumptions.

 

That would be a few "ten" thou. to the bore (actually about 8). We're not talking small block chevy's here.

I believe the bore is exactly 100mm, and its the piston that is undersize by .02mm.

 

Yes, the bore would be 100mm and the piston smaller.

 

Depends where you measure the pistons, they're not round.

S3 and S4 bores are 100.0, 100.01 or 100.02, and get group 0,1, or 2 pistons.

These are S4 pistons, group 1, not S3, but anyway:

Vertically, halfway down the skirt (62 mm below the crown), they're 100.00 mm (probably actually 99.99):



But they're only 99.41 mm wide at the same level




Vertically, above the top compression ring, they're only 99.47 mm:



And horizontally, above the top ring, 99.38 mm:

 

Are you saying they were oval from the factory?

 

Nah, these particular pistons had ~100K miles on them. They must be round when new, but that's how they wear with use.

Here's a used 968 piston from the last Zombie motor, just measured 'cuz I'm curious- It's also ovalled, at least at the top, adjacent to the crown:

Height 103.49:


Width 103.26:


I also measured some new pistons that may be going into the next Zombie motor- both are round.


JE: Height: 102.86



Width: 102.87

 

In order to compensate for heat expansion, all kind of pistons are shaped oval and conical (taper). Racing pistons much more than others. I did remeasure the piston shown here and the diameter at the very top is 99.42mm and below the oil ring 99.90mm. The other two dimensions are correct. Can´t understand why I measured incorrectly in the first place.
The aluminum heat expansion rate is high, the figures were taken at a temperature of 66F. If the figures had been taken at a higher temperature like 100F the piston dimensions would show an expansion by appr. 4,5/100 of a millimeter.
The 928 pistons are somewhat barrel-shaped, circular at the ring area but oval below the rings.
Åke

 

They are smaller on top to allow heat expansion.

 

Rob, I find it difficult to measure this kind of small tolerances using a digital caliper with great accuracy.
Using a micrometer will increase the accuracy quite a lot. However a digital caliper is probably the only tool we have available for the measuring of ring groove bottom diameters.
http://www.goodson.com/Micrometers-set/
Åke

 

Hi Åke-

Yes, I'm sure every good machinist has a small twinge of chest pain when they see pictures of my cheapass caliper, pretending to do precision measurement.

That's a pretty good price on that Goodson mic set- I troll Ebay on a semi-regular basis looking for a good Mituotoyo 0-6" set (and a bore gauge...), but they're expensive.

 

Rob, go for the micrometer set if you find the price good. This is a lifetime investment and I do not think your interest in powerful engines ends when you get older. Well you need bore gauges for the measuring of inside diameters too.
http://www.goodson.com/FBG-7-FBG-146...r-Bore-Gauges/
If you have the micrometer set you can do good with a set of telescoping gages which is only a fraction of the money.
http://www.ebay.com/bhp/telescoping-gauge
For the measuring of valve guide inside diameters this is what you need.
http://www.ebay.com/bhp/small-hole-gage
Åke

 

thats right!!!! i forgot.. the bores are 100mm, but the pistons are not even round! (and a few tenths smaller) (although my stroker had a few thou gap, not the porsche gap. )

 

The piston to cylinder clearance is between the largest diameter measured on the piston and the cylinder diameter. Next time you have your stroker race engine apart, it would be very interesting if someone with great precision could measure the cylinder bores at six different points, at the top, in the middle and at the bottom of the bores parallel with the wrist pin and in ninety degrees to the wrist pin. I am sure you will find a certain out of round and taper in every cylinder.
Åke