usually they break the dowel pin land that is right above there, because that is the closest torque constraint. I don't know that Ive ever seen one break the other dowel pin.

 

So when the block twists it is probably rotating on the bottom pin and binding on the top?

 

My guess is that it stays pretty solid until it breaks the top pin land, which is closer and weaker than the other one, then it rotates on the bottom one.



Japan2LA, do you generally have any thick center irons in stock? If I break this again before winter, or if I have any extra money when I go to stud it, I might be interested in replacing the irons... I generally face my irons on a surface grinder, so if you have one that is somewhat worn for a lower price that would be even better..

 

My thoughts were on spreading out the force that is exerted on the one pin near the oil pedestal. I believe they crack there because the strength of the materials and design can't take a whole lot more force. Even though the force is more concentrated on a short dowel rather than a long, you would be diluting the force through the fact there is more of them.

I believe some things come into play when you open that tension rod hole such as more weakening but that has proved to be overcome with the larger studs.

I thought a person would not want to dowel pin around the spark plugs because you would want some give in that area. I thought I read in another thread about coolant seals that the Mazda engineers designed the coolant seals to take flexing of the rotor housing. I thought I also read that the dowel pinning studs from Mazdatrix have .008" clearance. Seams like a lot and maybe for a reason.

RXBeetle has a good thread on reinforcing just the area around the factory dowel. Search "Adding Housing Dowels" and I think you will find it. It seems like a good idea but you have to make the dowels. I'm trying to go off the shelf.

 

Can you give me some more info? I'm doing that very same thing. We just use a general purpose wheel in all our big surface grinders. What wheel are you using? I only have a 36 grit H hardness AO wheel at my disposal. Do you just use the ground finish or do you do some other surface finish prep?


I screwed up in the last post when I stated that dowel pinning was the strongest. I was thinking of the tension bolts that are center less ground and fit closely to a machined hole through the whole assembly.

 

Here's RXBeetles thread. https://www.rx7club.com/rotary-car-performance-77/adding-housing-dowels-607542/

 

I am not sure about the wheel. I have used both a CNC surface grinder and a horizontal bench grinder (used to deck heads at the shop that has it). In both cases I go over the housing with 320 grit sandpaper for about 20 minutes/side to get rid of any sharp scratches afterwards. This seems to work just fine for me, I have done it on several engines with good results.

I liked the surface grinder finish better, but I no longer live close to that shop and the place I go to now lets me use their grinder for free...

 

fyi, i just got the car back up and running yesterday. I did end up stress relieving the iron, seems to be working fine so far. I'll post some pics in the 20b section, and I'll let you guys know if it breaks..

 

Stud it & also run a solid top dowel the full length of the motor. You will have to externally oil feed it into each bearing but it is worth it. They always crack at the top dowel hole on the fat plate because mazda left a gap there to get an oil feed to the centre bearing. It only takes the slightest movement to crack it, which is also why I think mazda used the stepped dowel pins, so it actually could move without cracking!

 

How'd that "patch" work out for ya ?

 

worked fine, i overboosted and broke the rear iron a while after that, but it has been up and running for a while since.

 

Did it break in the same spot ?

How much are you boosting now ?

What was your method of fixing it ?

 

top dowel land, yes. that is the one that always breaks. the one on the center was for the center rotor side tho, so it wasnt the same actual dowel.

i am boosting 17psi usually, but i made over 21 (thats all i can measure with my map sensor) the night it blew, due to a wastegate failure.

i replaced it with a fd rear iron modified to work on the 20b.
i also added some 10mm 4140 studs to try and hold it together a little better.


i plan to add an oil pan brace real soon, and next time the motor comes apart it will get drilled and studded with 1/2" 4140 studs all around.

also i think heat has a lot to do with this, as both times the engine has been pretty hot when i broke it. I am adding another oil cooler and doing a bunch of ducting to try and help with that.

the other thing is i am running 10" slicks this year, and im upgrading drivetrain parts as i break them, so i doubt the car will last long enough to break the engine very often till it all gets swapped out.... I gotta get some sponsors or something if i am gonna take it any farther.

 

The caveat of the Mazda 13B rotary:

The dowel lands on the front, center, and rear irons hold 100% of the combustion force that is made in the chamber with the major focus being on the upper dowel run and not the lower. It's funny when you look at it that way -- those dinky little pieces of iron that the dowel pins slide into hold *everything* we're throwing at these things.

Distributed load -- stagger off the sheer load to different portions of the housings. I really liked the idea of ~12mm, shorter "pocket" pins similar to the pinning used to index heads to bottom ends or bellhousings to engines. Non-intrusive, backward compatible, and would do the job as well as a 16mm dowel pin that leaves razor-thin pieces of rotor housing leftover.

B

 

^ and 20b too

i thought about that, but i was worried about bearing stresses on the pins, so i figure i will just stud the things in the future...

 

I've thought a lot about that method also. As a machinist I have seen "Swiss or hollow dowels" used in a lot of stuff. The problem is there's just not a lot of meat there.

The factory clearance holes for the tension bolts are all over the place. Going from .450" to .490" in dia. and sometimes are a conglomeration of partly cored and partly machined. Many visually out of round.


I think the minimum dia. would have to 14 mm to kleen up around the hole. I'm unfamiliar with metric but I would assume they would have a 10 mm ID. That might pose a problem. I will see what kind of info I find at work tomorrow. Flu day today.

Instead of going to 16 mm I would go english. A 5/8 dia hollow dowel has a 1/2 ID and is readily available. I just think it is to big.

I have to ask were you thinking of solid dowels not around tension bolts. If so never mind.

 

The Puerto Rican's I've seen use a solid pin, threadded on both sides (designed to thread into the front plate and then lock down in the back w/ a nut and crush warrrsher), 16mm wide, that requires the same type of pinning job as 'traditional' dowel pinning, but is also a tension bolt replacement. The one thing I like about this method is it distributes load to the adjacent combustion chamber as opposed to traditional pinning which still keeps it on only the adjacent housings -- this is effectively double the length.

B

 

yeah but then you ahve to run external oil lines, which is another possible failure area...

 

Since I'm home with the flu I did some research. this is for people who may be interested in adding hollow dowels around the tension bolts. I just thought you guys might be interested in the information.

I found three possible applications.

The most interesting so far is McMaster Carrs' offering. They offer precision ground hollow tube made of 4140 and 4130 steel with an OD of 9/16 (.563") and an ID of 7/16 (.437") in 6", 12", 36", and 6' lengths. You could theoretically run a hollow dowel through the whole engine or at least a copy of the factory idea and run through the housing to engage each side plate. The .063" wall thickness leaves a little to be desired but would offer alot of resistance if in a close tolerance hole. Especially if multiplied. They listed tool steels that can be hardened including S-7 (read RXBeetles thread) but could find no specifics.

The next interesting one I found at Misumi USA. They are a component supplier a little more on the technical side than McMaster. Look under automation components and then Linear Bearings/Bushings/etc. They make some metric bushings (part# prefix JBAU) from bearing steel at 58 Rc, 13 and15 mm in dia. with a 1.5 mm wall. Max length is 20 mm (~3/4"). These could be used without reaming through the entire housing. If you so choose. I re-read RXBeetles thread and Patmans take on the twisting dowel would come into play here. I think 3/8" into both the plate and housing would definitely help out a mildly prepped engine. Working in a machine shop and having access to CNC equipment I am seriously considering this even though it is a .060" wall thickness. The cost was about $12 a piece.

Last is another Misumi component much like the other except thicker walled (part# prefix JBAFMPG, same catagory as the other). 15 mm OD, 10.5 ID up to 40 mm in length (~1.5") You can choose between O1 tool steel, 440 stainless, or 304 stainless. The O1 and 440s are hardened. The only drawback is there is a .5 mm groove machined around them for retention in some applications.

 

I'm not talking about replacing the stock dowel pins with solid pins. I'm talking about replacing a few of the tension bolts with solid pins that act as both tensioners and additional dowels. No external oil line needed.

B

 

aka studding?

thats what i plan to do with the 1/2" rods as mentioned. I'm a little unsure of what clearances to use tho...

 

Yes, I have mentioned this in the past, making a tubular dowel out of 304 stainless steel about .060" wall thickness and about 1" long. The ID would be about .010" bigger than the tension bolt threads. Since they would only protrude into each side of the housing about 1/2" the leverage on each pin would be much less than on long pins and would be distributed around each tension bolt location.

I was originally going to attempt this on the engine I was building since I worked for a race team and had access to anything needed to do the machine work and I didn't want to spend $400 on studs. I got layed off though so have never been able to try it to see how it works.

 

I looked at the Misumi components closer and found a bushing in the same category as the ones I previously posted. It is a 15mm OD hollow bushing with a 10.5mm ID available in lengths up to 40mm. No retaining groove. That's a wall thickness of ~.088". Part number JBAFB. Also available in 440 stainless and 304. The 440S is hardened to 50-55 Rc. The JBAFB basic is O-1 hardened to 56 Rc. 304 is soft. Price for the O1 basic bushing is $10.22.

15mm = .5905"
10.5mm = .4134"
40mm = 1.575"

 

You guys could ask for miracles while your at it, how much more can Mazda rotarys give and be just a little more reliable. WHEW! just an opinion. Ron

 

^wtf are you talking about?