Arias Pistons - Bore size

[campbellju]

I've got some 86.5mm pistons to fit in the block. As they are American they are sold as 3.406"

These are the only spec details I can find. I would assume the pistons are designed to fit a 3.406" bore size (85.5124mm) so that is the spec I should give the machine shop?

I suppose this could be double checked by the machine shop by measuring the piston and adding 0.0035" (0.08mm)?

Don't want to do this again so thought it sensible to check.

Jim

 

[The Doc]

Need to know which grade of alloy your pistons are as arias uses two differant ones, which have two differant clearences.

 

[campbellju]

Thanks Doc.

I believe they are the non-silicon type but will double check tonight when I get home, there may be a star hidden away that I didn't notice.

http://www.ariaspistons.com/pdf/automotive-installation.pdf

 

[campbellju]

Just quickly checked the pistons. No obvious marking to denote 4032 alloy so I guess they are 2618.

Checked the new rings I got which were for 87mm pistons, measured the pistons with some digital callipers and it looks like they are 87mm too!?!

I'll take it all down the engineering shop this week and get them to double check what I've got.

 

[campbellju]

The pistons are 87mm and the material is most likely 2618. I'm trying to confirm with Arias but they never seem to be around when I call.

Machine shop will sort out the bore size if I can confirm the bore clearance. Internet is fill of opinions. Anyone got experience or shall I persevere with Arias?

 

[The Doc]

One thousandth of an inch per hundred horsepower is a rule to follow. So you would want 45 thou. Check with arias but I bet they state 35-40 thou Clearence to wall and the ring gaps of 19 thou top and 20 thou second ring. Do double check this as there will be a batch number on the piston to confirm these sizes.

 

[campbellju]

Thanks Doc, everything I found on the internet pointed towards 40 thou. I like your rule of thumbI'll continue to try and call Arias to check sizes.

Cheers,
Jim

 

[campbellju]

Doc, need you help again

Finally got in touch with Arais. I tried a different department in the US after never getting anyone to answer the phone at the correct department for the past few weeks. The lady picked up immediately and was very helpful.

They've confirmed it is 2618 alloy and a 3.425" piston designed for a Nissan turbo car. They recommended a 40 thou piston to wall clearance.

The new rings I have are number 1515403425. She explained this relates to 15 first 15 second and 40 third ring for a 3425 piston. She was worried that there may be too much gap.

Not being great on these things I was sure what she meant so I asked he if it was the ring gap, she said no, it means 1.5. 1.5 what I'm not sure?

She was worried about the end gap being too big and suggested these are checked in case we needed to get new rings.

So the pistons sound good, I can get those to the engine builder on Monday. The ring gaps, I haven't the foggiest what she was on about or what the numbers mean?

 

[campbellju]

I'll have a go at answering my own question.

I found this interesting general info on ring gaps:

http://www.aa1car.com/library/ring_end_gap.htm

I expect the 1.5 was a misunderstanding and they are really 0.015" end gap.

So the rings are just too small rather than just too big.

 

[The Doc]

I would go with 19thou top compression ring,20thou 2 Nd compression ring and 40 thou for the two oil control rings. That's what the 15 15 40 means , I always run larger ring gaps due to the extra heat from pushing the engine harder than was intended and I use a larger gap on the second ring so that if there is any blow by on the top ring it will get past the second ring quicker and help to prevent the ring land over heating and becoming damaged.

 

[The Doc]

I would not worry if the gaps are larger than needed within reason, I would worry if they are too small as the ends can but up and the ring then shatters.......in an ideal situation the pistons should have such a minimal Clearence at full operating tempreture that you create the perfect seal with out seizing and the rings do a minimal job then, hence why in a lot of high revving race engine they use low tension very thin rings to reduce friction loses but they have very tight piston to bore Clearence to reduce blow by to a minimum.

 

[campbellju]

Thanks doc. That all makes perfect sense. So the pistons are holding most of the compression, the rings are just sealing the edge. Hence why the ring gap is about safety first.

I feel all motivated again with the car. Might get it running again this year

 

[The Doc]

Excellent, next thing will be what cams are you running and what exhaust housing on the turbo

 

[PobodY]

Standard cams, but adjustable gears on them; if I remember correctly?

 

[campbellju]

cams are uprated with adjustable gears. I've got another head I might get rebuilt though. Turbo will be the same 3071 but target about 400 rather than 450hp. Aiming for a smoother power delivery.

The only what next is the HG. Last time I just skimmed the block and head with a 1.2mm gasket. no problem in raising compression a little for off boost response but still targeting around 8.5

 

[The Doc]

Cosworth is the only gasket for this engine, it comes in 1.3 mm I believe and you will want the 87mm bore, get the head worked properly as well the standard exhaust ports are awful and need a lot of work and the seats can be blended in and slimed down too......you will get far better response and the turbo will do less work if the engine is more efficient .

 

[campbellju]

A quick check on the piston to wall clearance. I started talking in terms of 40 thou. checking back through my notes with the Arias sales person, they were talking 0.004" which is 4.0 thou. 40 thou might rattle a bit too much

I was brought up on the metric system so get mixed up in imperial.

 

[The Doc]

My cock up too, 1 thou per 100hp is 4 thou 0.004, which is arias, wieseco are 0.003 and cp are 0.0035.

 

[The Doc]

Standard compressed head gasket thickness in metric is 1.2 mm in answer to your other post.

 

[campbellju]

Ooh. In that case I'll let the engine builder test build the engine first before decking anything.

Thanks doc