Hand lapping cast iron side plates

Technical info not specifically 1st gen RX7 related.

Hand lapping cast iron side plates

Postby TOOL » Thu Dec 26, 2013 5:12 pm

Hi guys,

Here is another thing I've been wanting to try out; hand lapping of cast iron side plates.
Again, this isn't something I could find out a lot about on the interweb, and I don't know anyone who has done it. I've done this as a little project, to see how hard it would be, what you need to use and what sort of result can be achieved.

Take a set of used cast iron side plates. These are some 12A S2 or S3 side plates (all N plates) with a reasonable amount of wear on them. I took the worst set of plates I had, with the deepest groove from the corner seals on the combustion side. Still useable imo.

Remove any dowel pins, orings, bolts, stat gears etc. so that you have a completely flat surface and nothing is sitting proud of the running surface. Give the plates a good clean so that crud in the water galleries won’t fall out while you’re grinding and get the worst of the rust etc. off the water seal area with a box knife blade or similar.

The front plate which has not been touched:

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You can see how blackened the metal is around the water galleries, some rust on the face and especially how shiny the seal surface is.

Secure the middle plate to something. I’ve used a piece of board with 2 bolts stuck through it that fit snugly in the dowel pin holes. The plate is not held down, it just can’t wriggle around.

Get yourself some grinding paste and place an amount on the plate. This is where the trial and error comes in. The first paste I used was too fine. It worked, but it was very slow. I took a coarser paste and it worked much better. You can them go back to the finer paste at the end once you’ve got your desired surface.

Take a second plate and place it on top. Grind the plates together (lapping the surface of the 2 plates at the same time), making you sure you rotate them regularly and you cover all the plates surface, including the legs. The plates are the flattest surfaces available so it makes sense to use them against each other.

After about 45 minutes working the plates together they looked like this.

Here is the plate with the grinding paste on it:

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Here is the middle plate, showing the grinding paste and the plate surface with the paste removed. You can see the low spots (the shiny bits) on the combustion stroke.

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Here is the untouched front plate in the background with the middle plate in the foreground. You can see the low spots (the shiny bits) on the plate:

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After about another 30 minutes using the coarser grinding paste, the middle plate came out like this (I haven’t removed the paste from the rear plate yet, and I haven’t used the finer paste on either plate):

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You can see the low spots at the top and bottom (the sides) of the plate and the corrosion where the water seals have been pressed against the plates. You don’t necessarily want to remove so much material that it is perfectly flat.
You can also see that the shine has been removed from the plate, meaning that you have a much better surface for bedding in new seals, making break in easier.

To highlight the improvement, here is a NOS 12A plate for comparison:

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The NOS plate has some marks on it from the protective coating Mazda used, but this has only been removed from the packet and box for the photo. You can see the difference in the metal from where Mazda hardened the centre for the oils control rings to run on.

Here are the NOS plate and lapped plate side by side for comparison:

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Here’s all 4 plates together; from left to right untouched front plate, rear plate with grinding paste on it, lapped middle plate and the NOS plate bottom right):

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It’s a pretty tiring doing the lapping, but you can get good results and you can control how far you take it, rather than hunting around trying to find someone you trust to lap your plates.

Before using the plates, they will need to be very well cleaned to get all the paste off otherwise it will cause severe damage in your motor. This is no different to when you port a motor or simply assemble a stock engine.
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Re: Hand lapping cast iron side plates

Postby ian65 » Thu Dec 26, 2013 7:46 pm

Great post Tool!
The part of the plate that Mazda hardened for the oil control rings to run on....... I take it that's the lighter/shiny bit on the face of the plate, around the centre hole?
Does lapping the plate remove this and if so, does it matter? I suppose lapping the plates is similar to glaze busting the bore on a piston engine to allow the rings to bed in?
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Re: Hand lapping cast iron side plates

Postby TOOL » Thu Dec 26, 2013 9:15 pm

ian65 wroteColonThe part of the plate that Mazda hardened for the oil control rings to run on....... I take it that's the lighter/shiny bit on the face of the plate, around the centre hole?


Yes, that's the part I'm talking about. However, I think what I should have said was that the metal around the centre is somehow different to the remainder of the plate. I can't imagine they only hardened parts of the plates. It's an insert of some description and is presumably harder for the oil control rings. IIRC :?

You can see them here on a set of twin dizzy plates I had surface ground (not lapped) about 4 years ago when I lived overseas:

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You can see how much more shiny the plates are in comparison to the plates I hand lapped.

You can see in these photos that the 2 metals are completely different:

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Strangley, the middle plate didn't have it:

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And these are the plates before surface grinding:

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Note that you can't really see the 2 different metals on the plates prior to surface grinding. I only saw it when i went to pick them up after being done.
This motor had been sat for a number of years, hence the condition of the plates.

I know a UK Mazda dealer from the 70s. I talk to him from time to time and he was telling me that Mazda made the plates in 3 sections iirc. One was the centre where the oil control rings run and I can't remember the other 2, but assume it was remainder of running surface side and other side.

ian65 wroteColonDoes lapping the plate remove this and if so, does it matter? I suppose lapping the plates is similar to glaze busting the bore on a piston engine to allow the rings to bed in?


The lapping process is designed to take the bare minmum off the plates, so lapping is preferential to surface grinding to maintain the nitride surface. My twin dizzy plates aren't surface hardened (and didn't use hardened oil control rings) so I wasn't going to lose something that wasn't there in the first place.

The lapping can also be likened to honing a cylinder bore, to take the shine off it. It will aid the bedding in of the side and corner seals and give a flatter running surface too.
It's the same principle.

I think it's well worth doing as part of a build. Bear in mind, I don't know anyone in the UK who is building a 12A and using new parts, other than springs and seals. The 12As are running on borrowed time imo so removing a little nitriding isn't going to hurt them. My guy also says that it was only 1984 when the nitrided plates started coming through as well, so that's only the S3s.
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Re: Hand lapping cast iron side plates

Postby biznatch » Sun Mar 23, 2014 2:49 pm

Great post Tool, thanks.
It makes a sense to give the seals a fresh surface to wear into and to flatten the surfaces a bit this way. I think I'll give this a go on the 12a im porting at the moment as my plates have some wear from the corner seals and I'd like to reduce this even if I don't get rid of it entirely. I'm not too worried about loosing some of the nitride surface this has probably already been work through in the critical areas anyway (ie corner seal tracks)
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Re: Hand lapping cast iron side plates

Postby TOOL » Sun Mar 23, 2014 6:17 pm

The 12A plates are definitely much softer than FC plates.

You see much less wear on FC plates, even with the addition of boost.
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Re: Hand lapping cast iron side plates

Postby TOOL » Thu Jul 17, 2014 9:40 pm

Here's some FC plates I'm working on.

Centre plate before:

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Front plate after 30 minutes:

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Not too much so you don't remove all the marks or cut through the hardened surface.

Here they are side by side:

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You also don't want to take too much off and affect the seal groove depth.

The ports have all had a tickle too.
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Re: Hand lapping cast iron side plates

Postby kenwhiteside » Fri Jul 18, 2014 3:45 pm

I would have thought the difference on finish you saw on the ground plates is the effect of the hardening/nitriding on the material rather than an insert of a different material. The hardening/nitriding process will change the grain structure, which is why it is harder, that may polish up different.

Have you built any of these hand lapped plates into an engine yet?
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Re: Hand lapping cast iron side plates

Postby TOOL » Fri Jul 18, 2014 10:11 pm

There's very little information on the net about the process Mazda used to harden the plates.

I've found references to a "3 step process" and "coil induction heating" but nothing much more than that. The guy I've tried to quote was a Mazda dealer in the 70s. He knows his stuff.

He said they were built in 3 parts, but maybe what he meant was the processed them in 3 different steps. I'm not sure how, as any heat treatment (Nitriding) I've seen discussed has always been on a whole plate, not individual parts of a plate.

I haven't used the plates to build a motor yet. The 12A plates were test plates and are sitting in the shed. The FC plates are for my 13B NA build.

I liken it more to honing a cylinder bore. I've taken the high spots down and exposed the low spots, knowing full well that this would be where the corner seal runs past the plugs. It should also give a nice surface for the new seals to bed into.
It also cleans up the mating surfaces which can only be a good thing.

I could take it further down with more grinding, but then I will eat through the hardened surface and even risk squashing the water seals as the groove is shallower.

Here's a nice link:

http://www.mazda.com/stories/rotary/how ... rotary.swf

You can see some localised hardining of the rotors in the second video.

And a link to some rotary engines in planes:

http://www.knology.net/~rv7rotary/Rotor ... riding.htm
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Re: Hand lapping cast iron side plates

Postby re japi » Sat Jul 19, 2014 4:03 am

Awesome post, thanks W(p
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Re: Hand lapping cast iron side plates

Postby spirit r » Thu Jul 24, 2014 8:09 pm

Some advise from Racing beat:
Side Housing Resurfacing (Lapping) - When rebuilding your engine it is highly recommended that you resurface the side housings if the level of wear permits. We recommend removing no more than .002-inch of material from each side of the 1986-95 housing surfaces to maintain the integrity of the O-ring grooves. With the 1971-85 housings we can remove material to the following minimum thicknesses: Front Housing 1.568-inch; Intermediate Housing 1.955-inch; Rear Housing 2.355-inch.

@Tool :Thank you very much that pictures are perfect for me cause of my english defizit
Thomas
_________*Thomas*


http://www.rx7fb.com/viewtopic.php?f=3&t=1116
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