NB8A, BP4W engine rebuild

wydra · Postby **wydra** » Wed Apr 29, 2015 12:39 pm

I saw that bottom end, sounds perfect and I would purchase it IF it had lower compression pistons. 11:1 is too much for FI on 98

zossy1 · Postby **zossy1** » Wed Apr 29, 2015 6:02 pm

If you run a 0.070" or 0.120" cometic head gasket and kept the boost at 12psi or less, you'd be fine

Any more boost than that and I'd be a little concerned but more with the 85.5mm bore which is a little big for high boost applications.

wydra · Postby **wydra** » Wed Sep 09, 2015 2:58 pm

Hey guys,
I thought I would give an update with where I am at.

The engine has been rebuilt with Supertech forged pistons and Manley forged rods. I had the block re-bored to suit, everything measured up and everything balanced.
The engine is now back together, back in the car and I have done the initial break in period. I changed the oil and filter at 100km and am now running the second batch of oil, currently at 250km. I plan to run this oil/filter for about another 500km and will do a final drain and fill it with synthetic 10W-60 and a new filter.

Everything is going great so far, except for a sound...
At first I thought I was going to have to pull the engine back out and start over again as I thought it was a big end knock. I have heard this noise in the past and it sounds quite like it (varying between throttle and vacuum between about 2000-3000rpm).
I contacted my machinist and he assured me all the bearing surfaces and bearings were measured and within spec and due to installing the forged pistons, it was probably piston slap.
The noise goes away when the engine is warm which does indicate it probably isn't the big ends rattling.

Anyone else had anything like this with forged pistons?

At this stage I am going to keep driving it for the rest of the break-in period and see how it goes. Obviously if it gets any worse or starts happening when the engine is warm I will stop straight away.

Black_Penguin · Postby **Black_Penguin** » Wed Sep 09, 2015 3:19 pm

Forged pistons will rattle when cold. Just the nature of the beast.

wydra · Postby **wydra** » Wed Sep 09, 2015 3:48 pm

Black_Penguin wrote:Forged pistons will rattle when cold. Just the nature of the beast.

As long as it is just this, I am happy to live with it

When I first heard it and thought it was big end knock, my heart sank. I am still not 100% certain but things are certainly pointing towards piston slap/rattle.

project.r.racing · Postby **project.r.racing** » Wed Sep 09, 2015 5:54 pm

It is a pretty common gripe with forged pistons. Google would turn up millions of results.

StillIC · Postby **StillIC** » Wed Sep 09, 2015 6:31 pm

Black_Penguin wrote:Forged pistons will rattle when cold. Just the nature of the beast.

Not exactly.

The fact that some pistons are forged is not what makes some forged pistons rattle when cold. It is the fact that they are made from an alloy that has a different coefficient of thermal expansion (COTE) to the block they are housed in.

Supertech 4032 forged pistons should *not* rattle when cold if installed correctly, as this alloy has a COTE almost the same as the iron in the block, hence why the clearance should be about the same as for a standard piston. But I wouldn't personally use 4032 in a turbo engine with any significant boost.

Forged pistons made from 2618 alloy are stronger than 4032, for the same weight, but they have a different COTE to iron, therefore requiring larger clearances when cold, so they have room to expand into when hot. These will rattle when installed correctly.

wydra · Postby **wydra** » Wed Sep 09, 2015 6:41 pm

Google did come up with a million results on it, hence why I called my machinist and came to the conclusion it "should" be piston slap.
I also see a lot of people diagnose it as big end knock so I thought I would throw it to the forums, see what some other MX5 owners have personally experienced with forged pistons.

All good

I'll keep pushing on and keep an eye (and ear) on everything.
Besides the cold engine noise the engine is running awesome.

noobee · Postby **noobee** » Wed Sep 09, 2015 7:09 pm

your machinist will know what clearance he machined it to,so he will know whether they are a loose fit or not and whether to expect piston slap noise.
I had this problem many years ago with a set of forged venolia pistons in a ducati (before I became a machinist myself).I did not provide the engine shop with the machining specs,so they machined them quite loose,around .006" or .0007" from memory,should have been .00015''.I ended up getting it resleeved and rebored.
Mick (now the ex machinist,lol)

wydra · Postby **wydra** » Tue Nov 10, 2015 12:56 pm

Are there any guru BP guys out there that might know what this is?

Since my rebuild the engine has been running great and has since put 1500km's on it with a few oil changes.
When the engine is not at full operating temps there is still a small "rattle"/"knock" that occurs between 3000-4000rpm. Its hard to explain, it sort of sounds like a very faint big end knock. To hear it, I literally need to be either on the freeway at a slower speed next to a wall with the windows down or in a tunnel with the windows down but it is there.
When the engine gets up to full operating temps, these noise goes away and the engine sounds as it should.

Mr Morlock · Postby **Mr Morlock** » Tue Nov 10, 2015 3:38 pm

I don't think that the noise can be diagnosed by anyone sitting behind a computer X kilometers away. I would think that you would need to see an engine specialist who could provide an opinion. They may then be able to give advice as to what to do next. I also think that you pay for the advice and it might be taken seriously. Someone on the forum might be able to suggest some specialists in your area.

wydra · Postby **wydra** » Tue Nov 10, 2015 4:45 pm

Morlock,
I have taken it down to my machinist/engine builder and went for a drive with them to asses the situation.
He believed it was piston slap and not a terminal bearing issue. He also came to this conclusion as the noise disappears as the engine gets up to temperature. I went to him with this noise when the engine only had limited

I was simply throwing the question out to the forum to see if anyone else had the same or similar issue and had it correctly diagnosed. I don't want to pull the engine out and apart for no reason but also don't want to leave the engine in with a issue.

My next port of call will be down to the dyno so I can load the engine up while it is stationary to try and diagnose it better.

madjak · Postby **madjak** » Tue Nov 10, 2015 7:23 pm

Maybe drain your oil and have a look. When my big end went you could easily see the shavings in the oil... it's probably the easiest way to rule out a bearing issue, and if the oil is good, you can be cheap and pour it back in.

But I doubt it's a big end, you'd hear it whine at high revs and the tick is very loud at idle / slow running. It's very hard to miss and will also get a lot louder quickly.

wydra · Postby **wydra** » Wed Nov 11, 2015 6:28 pm

madjak,
I have been changing the oil frequently and there are no particles to speak of in it, there has been a shimmery metallic substance suspended in the oil but as I am bedding in new rings, assumed it was that. I am due to change the oil again soon so will have another inspection.

Today when driving the/a noise all of a sudden got dramatically worse. About 3 minutes later I popped the bonnet and managed to get this short video with the "rattle" although it wasn't as bad as when it first started.

This morning when I restarted the car expecting to hear a rattle, it sounded fine. I let it warm up for about 10 minutes and still nothing so I drove it into work and it is running fine again.

I am going to try and take it back to my machinist/engine builder tomorrow and see what we can find out.