I need to get my side plates lapped and re-nitrated, and already found some professional companies that will do that - unfortunately, the have no experience with rotaries itself and even i trust them and they know what they are doing, i thought i would be great to give them some specs they can use...

1. Any specific quality of the lapping disc? Diamond plated?

2. to which surface quality should it be lapped? I don't know if Mazda every disclosed some thing about that...

3. re-nitrating - gas nitrating or which way is best? Which surface quality?

Hopefully any of you guys can bring some enlightenment...

 

heat nitriting is best but it also requires all freeze plugs be replaced and all brass drift plugs in the irons be removed, threaded and plugged.

lapping should be done to the finest spec that the machine shop can manage, the finer the better with the lowest final porosity being ideal. ie, the mirror finish you see on used irons is actually what you really want to have as quickly as possible.

 

Watch out how much you remove from the iron! 2nd and 3rd gen RX7's have the grooves for the sealing o-rings in the plates. When too much metal is removed the grooves get too shallow causing all sorts of problems with the waterseals; Pinching during assembly, not fully seating the plates together or cracking the material between the inner groove and the waterjacket.

Racing beat recommends that a maximum of 0.02" (0.05mm) can be removed. With the housings I've lapped that is usually just barely enough to remove the wear marks. So measure the housings before lapping and keep track of how much material is removed.

 

they have but you need a 70's shop manual for that

 

....thanks for the infos guys!

I might be wondering, but would't the 13B-REW plates different to the 70's rotary engine? i think the nitrading would be different (if the old plates needed to be nitrated, of course)...

I just checked my second set of plates, and the rear plate has on scratch on it, right below the port, perhaps 2cm long but at least 1mm deep - that might come from a broken/stuck seal, can it be repaired prior to the grinding/lapping, or is it cheaper to buy a new one?

 

up to 1978 the plates are a like a nice grade of iron, and then NOT coated. since the water seals are in the rotor housings, you can just lap them flat. the manual tells you how to do it and what surface finish to use.

after 1978, with the introduction of the Rx7, the plates become nitrided, and you can't lap them anymore. the nitriding, in addition to slowing the wear WAY down on the irons, and protecting them from rusting (its the grey color on the outside, vs the black paint of the old ones), also reduces friction by a lot. also since there is a coating, the base iron can be less special.

so lapping a 79+ plate is kind of a last resort, or if you were experimenting.

 

he did mention nitriding them, which should be done whenever lapping any RX7/8 iron, the stock nitride layer is only .040" thick supposedly. in my experience that number is actually inflated. once step wear is out of spec, the nitriding is already to the layer you would need to lap to.

 

...i read somewhere that re-nitrating is necessary if at least more than .003" are taken off... But can you be sure if the nitrate will still be intact otherwise? Nitrating is always a good idea i think...

The only problem about that is, there are MANY ways to nitrate...chips is doing gas nitrating, BUT that needs very high temperatures...several hundred degrees over some time (and not only on one spot during combustion in the engine), as the plates are simple cast steel, i might be wondering if you have to take care of anything or use a special nitrating...