Ownership Verified: We can rebuild it... we have the technology (7 - take two)

Received a bunch of parts, including new piston and both valves for #4. The piston has rounded inner edges of the chamber, unlike mine. And the inlet valve has an undercut stem, while mine are equal thickness. Like this:



I don't know if this changes flow properties or something, but at least the weight will be different. Is that bad?
 
The weight of the valves is irrelevant, it's a too small amount to make any difference.
 
Could you post pictures of the differences in the pistons? The rounded off edges are good for preventing heat spots, but as your engine effectively uses the top of the piston as the combustion chamber, the volume of the "bowls" needs to be exactly the same in all of the pistons as it affects the compression ratio of the engine. If the difference in the piston shape is only slight, it's quite ok modify the current pistons by hand (rough sandpaper, careful die-grinding) so that they have roughly the same dimensions as the new one.

How thick is the "waisted" part of the new valve stem compared to the thicker part that sits in the valve guide? The leaner stem increases flow, and as such is beneficial. Are the new valves the same material as the old ones? (top tip: You can test the material by checking how magnetic the valves are.) It's quite ok to chuck the old inlet valves on a lathe and make the stem a bit thinner if they are one piece stainless steel valves. However It's not absolutely necessary to do so. Did you check the valve lengths yet? Also, how big is the weight difference of the valves?
 
Ok, this is slightly worse than I initially thought. Now that I have both pistons in front of me, the difference is significant.

Both crown and skirt of the new piston are longer if I level them by the wrist pin and oil rings:



Here's the bevelled edge I talked about earlier, of course it's irrelevant now:


The little holes in the wrist pin bores are larger on the old piston:


Otherwise seems the same:


The pins are also slightly different, the inside of the old pin had a step where inner diameter decreases, the new pin has a smooth transition:


The weight difference is obviously significant.

Old piston: 353g
New piston: 363g

Old wrist pin: 88g
New wrist pin: 90g


I haven't even looked at the valves yet, but they are 15eur and not 150 like a piston, so I'm not that worried...
 
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nicjasno said:
The weight of the valves is irrelevant, it's a too small amount to make any difference.
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Did nicjasno just say something to do with a car was...irrelevant...? :eek:
 
When you put the pistons on a table side by side with the combustion chamber facing upwards, is the valve clearance/combustion chamber area at the same height from the table top, and are they identical in shape and size? If so, and the new piston only has a higher "squish" area, you can machine it down to make it identical to the other pistons. This is a perfectly acceptable method to make parts fit an engine, and is often done when decreasing the compression ratio of an engine. You obviously need a person with a lathe for this. Don't give the job to someone who hasn't machined one down before, as it's quite easy to damage the piston skirts when fitting it to the jaws of a lathe if you don't know what you're doing. Also, I wouldn't say that a 10g difference in piston weight is that significant considering the differences between the parts.

The wrist pins and wrist pin oil holes make no difference.'

EDIT: Just read that skirt is longer on the new one aswell. as such you need to measure the heigh difference from where the piston pin mounts.
 
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Posmo said:
When you put the pistons on a table side by side with the combustion chamber facing upwards, is the valve clearance/combustion chamber area at the same height from the table top, and are they identical in shape and size?
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Yep.

Posmo said:
If so, and the new piston only has a higher "squish" area, you can machine it down to make it identical to the other pistons. This is a perfectly acceptable method to make parts fit an engine, and is often done when decreasing the compression ratio of an engine.
Click to expand...

Yep again. I'm not sure if the design of the piston has changed, or has the original builder machined it. Strongly suspect the latter.

Posmo said:
You obviously need a person with a lathe for this. Don't give the job to someone who hasn't machined one down before, as it's quite easy to damage the piston skirts when fitting it to the jaws of a lathe if you don't know what you're doing. Also, I wouldn't say that a 10g difference in piston weight is that significant considering the differences between the parts.
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Sure, but I'd rather get it down to within a gram at the end.

Posmo said:
EDIT: Just read that skirt is longer on the new one aswell. as such you need to measure the heigh difference from where the piston pin mounts.
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Yeah, I actually connected them by a single wrist pin to do measurements. I think the crown was machined to achieve needed clearance/compression, and skirt perhaps for balancing?
 
Yes, both those assumptions could be correct. The thing you need to be worried about is machining the squish area of the pistons, the balancing process is simple DIY stuff, within a gram is fine. Most production car engines have differences of 2-3grams in both pistons and rods. If you get the piston machined by someone who knows what they're doing, you might aswell have the old valve stems thinned aswell?
 
Posmo said:
...you might aswell have the old valve stems thinned aswell?
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Ah, I don't know about that. I think I got wrong valves. Well, they are right for the engine, but they are an updated version.

The inlet, you can see the different cut. The old one had a slightly steeper back cut, while the new one has wider face and a lip of some sort.


The exhaust is almost the same as the old one, but again the lip is present.


And the stem tips are slightly longer, about 0.1-0.2mm I estimated.


And the Burton catalogue does list them as "Latest spec. With hardened stem tips". I asked them if older spec is also available, will see..
 
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MXM said:
Ah, I don't know about that. I think I got wrong valves. Well, they are right for the engine, but they are an updated version.

The inlet, you can see the different cut. The old one had a slightly steeper back cut, while the new one has wider face and a lip of some sort.


The exhaust is almost the same as the old one, but again the lip is present.


And the stem tips are slightly longer, about 0.1-0.2mm I estimated.


And the Burton catalogue does list them as "Latest spec. With hardened stem tips". I asked them if older spec is also available, will see..
Click to expand...


The old valves have been modified near the seating area. It's very common to backcut valves to increase flow: http://www.ws6transam.org/Backcut.jpg This is a very simple process again, can be done with either a lathe or a good pillar drill. The wider seating area (like in your new valve) is for road engines that need to last for 200k km's, for more sporty engines, the seat area is almost always thinner, as a thinner seat starts to flow immediately when the valve opens, as opposed to a thicker, non-backcut seat.

If the catalogue lists that they have hardened tips, then the new valves are stainless steel with stellite tips. The stellite tip is around 1-1.5mm thick, so you can only remove about 1mm safely without compromising the valve tip strength. (Though some people have shortened them more with no problems) If the length difference is only 0.2mm, it's completely fine to shorten the new valves by that amount.

What I would do in your position:

Take the entire engine to a competent machine shop, and ask for a quote for all the work, including:

-Piston modification
-Valve shortening, backcutting (and possible stem modification)
-A cylinder block hone (might aswell have it done in a machine shop since they can measure and correct possible ovality in the block. In the place I use they charge 10?/cylinder for honing, so if you're only doing a 4-banger, it's cheaper to have it done by a pro than buying a good honing tool.

Everything else you can do yourself, including piston ring gapping, valve lapping/installation, installation of rods/pistons, cylinder head, etc.

I have most (if not all) the special tools you will need, and can borrow them if you want.
 
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Thanks for all the information, Posmo, it's very helpful. Anyway, here's my latest information...

Piston:

The manufacturer never changed the pistons, engine builder has modified it to get the compression ratio right. Apparently this is a very common thing, and these pistons actually come as slightly too long for most applications. According to my quick calc the CR goes from 12:1 to about 11.2:1. So I will get it shortened. I called the machine shop, it's a quick job, shouldn't be more than 20-30eur.

Valves:

This is a bit tricky, the valve specs are different, the non-waisted inlets in this size are not available. There's is also difference in stem tip length and a back-cut on the inlet as illustrated above. For the exhaust I think I will just get the new valve stem tip shortened to match others and that's it. For inlets I have a few options:

1. Get 3 more valves of the new spec and have a back-cut made on all 4.
2. Shorten valve stem and make back-cut on the new valve, and machine the waist into the 3 old valves.

I'm leaning towards option 1, less machining, new parts, probably similar (or smaller) price.

But it also sounds silly to replace inlet and not exhaust valves, because it's exhaust valves which wear out more... 3 more exhaust valves are extra 45eur :p


Soon approaching a 4th digit, damnit!
 
Damn cars... so expensive.I'm slowly approaching the 5th digit on the e30 :)
 
MXM said:
Thanks for all the information, Posmo, it's very helpful. Anyway, here's my latest information...

Piston:

The manufacturer never changed the pistons, engine builder has modified it to get the compression ratio right. Apparently this is a very common thing, and these pistons actually come as slightly too long for most applications. According to my quick calc the CR goes from 12:1 to about 11.2:1. So I will get it shortened. I called the machine shop, it's a quick job, shouldn't be more than 20-30eur.

Valves:

This is a bit tricky, the valve specs are different, the non-waisted inlets in this size are not available. There's is also difference in stem tip length and a back-cut on the inlet as illustrated above. For the exhaust I think I will just get the new valve stem tip shortened to match others and that's it. For inlets I have a few options:

1. Get 3 more valves of the new spec and have a back-cut made on all 4.
2. Shorten valve stem and make back-cut on the new valve, and machine the waist into the 3 old valves.

I'm leaning towards option 1, less machining, new parts, probably similar (or smaller) price.

But it also sounds silly to replace inlet and not exhaust valves, because it's exhaust valves which wear out more... 3 more exhaust valves are extra 45eur :p


Soon approaching a 4th digit, damnit!
Click to expand...

Ok, Makes sense that the pistons were modified, and it's visible in your pictures that the Accralite part number is the same. The compression height of the old pistons has been lowered. I wouldn't touch the skirt of the new piston unless it's necessary for getting it balanced, as a longer piston skirt is always desirable as it makes the cylinder bores/piston skirts last longer.

Ring up Rallyrace.fi in Kerava and ask if they have any non-waisted stem valves for the kent engine. It's a bit more expensive to buy one in Finland, but if they have the type of valve you need, it would most likely be more economical to buy just the one valve rather than replacing 3 more.
 
Yadda yadda yadda..

OnT: Very cool seing this piece by piece, MXM! :D
 
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