I have some irons for my turbo 2 which are a little out of the flatness spec. I was told buy a friend who owns a machine shop that he could probably just mill them flat again and clean them up with flatness grinders. Is this an acceptable way to fix them? Not much metal would be taken off, it is removing about .003 of metal to get a step worn into the combustion side of the iron out. All 3 irons have similar wear.

What do you guys recomend?

 

Lapping the irons.

 

i would also recommend lapping versus a surface grinding, because surface grinding is not an even cut and will take off more material from some areas than others varied from the original true flat origination surface.

lapping is recommended because the nitrite layer is only about .015" thick.

.003 is not a specification so i assume he meant .030" which is well beyond the nitrite layer. if you need to take off .030 to get a it back within spec then the irons are either junk or need to be nitrited again, in most cases lapping and nitriting cost about $150-200 per iron.

 

Maybe he just needs 0.003" (3 thou.) taken off each side?

That's very little, but measure all the clearances after (rotor to housing, apex seal to housing, etc.) anyway.

 

where can you get it re-nitrite ? just wondering.

 

The first .004" (which is about .1 mm) of the nitride layer is the hardest and most important part.

 

that is less than i have found from researching on other nitriting measurements, basically the nitriting layer is already burned through on almost all irons with ~70k or more miles then. i still haven't noticed severe wear from lapping irons beyond that .004" figure though.

nitriting was introduced with the later model 13B engines(FC and beyond), early model 12A and prior engines were just milled cast iron and had a tendency to have about twice the wear of the later model engines with the same mileage. i wonder if nickelsil would be an alternative, of course we have neither in vegas. there are facilities in various places who can do either but only a few that can do both lapping and treating, i only do lapping.

keep in mind this was something new when it was introduced so Mazda's nitriting process may be inferior to today's standards.

 

This is not true. Surface grinding is very accurate and will leave you an almost perfectly flat and smooth surface. Most likely, too smooth for oil retention. Some sort of additional surface prep would be necessary (before re-nitriding).


The trick with the irons is the hard nitrite surface is difficult to surface grind without getting too much heat into the work. Even using coolant. One really has to know his stuff to get good results. The heat can cause the metal to expand and bow up into the wheel causing low spots.

I used a 36 grit "H" hardness wheel (14" x 2") and coolant. It was a bitch. You will have to constantly dress the wheel because the nitrided iron plays hell with it. One has to watch for plate warpage and shimming of the first side down might be needed. That or a variable magnet. Of course, a good stoning on the first side down is a must.

 

correction, i did find a shop locally who does nitriding after some digging and called for estimates.

keep in mind this is an oven that charges by the hour whether it has 3 irons in it or 20(maximum capacity for a small oven). a full day of baking is $700-800. so if i can get 20 irons together then the process is ~$40 per iron or if i only have 1 set of 3 irons to be baked it is ~$250 per iron.

i think you just proved my point.

 

nitriding was introduced for the 79 model year, so every Rx7 has a nitrided engine. the pre Rx7 non nitrided irons are painted black.

 

It depends on the quality of the company that you take them to. I used several here in AZ. The machine iIprefer for plates, has a stone about 12 inches wide and the bed travels about 6 feet. Also there is centerless grinding. And you can choose the RMS finish you want. Lapping alone is a hugely time consuming process that is usually used after grinding.
Don't know anyone who will "NITRIDE" used plates.

gd

 

It depends on the quality of the company that you take them to. I used several here in AZ. The machine iIprefer for plates, has a stone about 12 inches wide and the bed travels about 6 feet. Also there is centerless grinding. And you can choose the RMS finish you want. Lapping alone is a hugely time consuming process that is usually used after grinding.
Don't know anyone who will "NITRIDE" used plates.

gd