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Ya, crud is definitely normal especially if your motor was on its way out. You start having more blow by with oily crankcase vapors that make it into the intake path which end up gumming up the intake side a bit.
Awesome. Thanks guys. I'm glad the buildup isn't a terrible sign. Also very happy the cam lobes are normal as well.
Some more progress yesterday. Before taking the head apart I checked all the valve lashes. I believe the specs are the following (thanks Car Passion Channel)
Intake = .008 - .009 Exhaust = .011 -.013
And just for informational purposes I checked the roundness of one of the cam lobes on cylinder 4. In retrospect I should have check the exhaust side because those were out of spec. But the test setup wasn't ideal anyway and the values aren't really that accurate (I had to mount the dial indicator on the engine stand, not the engine itself).
Then I started removing the cam caps. Very thankful all the journals look healthy. No signs of damage.
Removed the head. Cylinder 4 is toasty.
Cylinder 2 isn't great either. Which is cool to me, because the compression tests showed 4 and 2 as the weakest.
Cylinder 1 wall condition: still see some cross hatching. No scratchs felt with the nail.
Cylinder 2 wall condition: no cross hatching. Very shallow scratches felt.
Cylinder 3 wall condition: Some cross hatching, no scratches felt
Cylinder 4 wall condition: No cross hatching, lots of shallow grooves felt with nail
Cleaned the deck and checked for flatness with a straight edge. Couldn't get a 0.0015 feeler under anywhere. So I'm thinking a decking isn't needed.
This scared the absolute **** out of me. I thought the block was broken.
Checked crank walk (my first car was a DSM btw). I measured 0.19-0.20mm which is about 0.0075". Max is 0.012 so I think we're good there.
Checked connection rod gaps. All within spec (0.0044-0.0103)
Removed pistons and caps to inspect journals. All the journals were good.
But a couple of main bearings showed signs of wear.
Between cylinder 1 and 2: no groves felt
Between cylinder 2 and 3: no groves felt
Between cylinder 3 and 4: groves felt
Last bearing: no groves felt
I'll be taking a closer look at the crank journal that had a scratched bearing. But it looks like a particle was introduced and ejected in a few rotations. Maybe. Im not sure. Otherwise I don't have any concern about the crank.
I just have to figure out if I should get the cylinders bored or just do a rehone myself. What's the qualifications for a required bore? Is it just any physical groves in the walls? If that's the case then it definitely needs to be bored...
Also, I was expecting to see some broken rings or some obvious signs of what caused the damage in cylinder 4 (and maybe 2). I couldn't find anything that stood out to me. Am I missing something? Is it possible I won't actually see anything out of the ordinary?
You could have just ran 4 too hot, ring gaps from factory are much smaller than a typical forced induction build. That’s only a guess though.
If you have access to a correctly sized ball hone, soak it in ATF and go up/down fairly quickly at a slow drill speed. If the grooves aren’t gone, it needs a proper bore and oversized pistons.
You could have just ran 4 too hot, ring gaps from factory are much smaller than a typical forced induction build. That’s only a guess though.
If you have access to a correctly sized ball hone, soak it in ATF and go up/down fairly quickly at a slow drill speed. If the grooves aren’t gone, it needs a proper bore and oversized pistons.
Interesting.
I'll get my hands on a ball hone and try it out. Thank you.
Originally Posted by emilio700
It sounds like you don't have an FSM (factory service manual). If so, please get one stat.
Hey. How'd you tell? I usually just search up the specs I'm looking for but I have these for reference as well found here.
Some further investigating. Used a dial bore gauge to check the cylinder diameters at the top middle and bottoms (all within piston travel range). X1 is parallel to the crank, X2 is perpendicular (I guess this would be called the trust axis). OOR is the out-of-round measurement - all within spec. All unites are in mm.
After taking all the measurements I went back and did some spot checking to make sure 1) the tool hadn't lost calibration and 2) I didn't mess up with taking the measurements. I could not for the life of me get consistent readings on cylinder 2. Is it possible that cylinder wall is warped or wavy or something?
Anyway, do you guys make sure the machine shops that work on your blocks use a torque plate? Overkill? I did a quick search and it doesn't seem to be too big of a concern but it's possible people just don't mention it.
I also just (embarrassingly) realized all the recommended forged pistons (supertech/wiseco) are for oversize bore sizes. I've been trying to figure out if the block actually needs to be bored but in reality, if I want to use forged pistons as planned, an overbore is necessary. If I understand correctly the minimum target bore size is 83.5 to fit the smallest commonly available forged piston. Good idea? Worth it? I have no idea. What would you guys do for a HDPE track car that will probably get a turbo eventually?
Last edited by HalalBuilt; 01-02-2024 at 05:33 PM.
Smallest pistons the block will take for F/I. Thicker walls. Early blocks have thinner walls and more core shift. NB2 blocks tend to have thickest walls. For anything above about 15psi, we suggest sonic testing the block to ensure at least .120 wall. .150 more bester
Smallest pistons the block will take for F/I. Thicker walls. Early blocks have thinner walls and more core shift. NB2 blocks tend to have thickest walls. For anything above about 15psi, we suggest sonic testing the block to ensure at least .120 wall. .150 more bester
You're saying I should run standard/oversized (0.010" or 0.020" over) OE pistons? Or smallest forged pistons the block will take (83.5mm is what I'm seeing)?
You're saying I should run standard/oversized (0.010" or 0.020" over) OE pistons? Or smallest forged pistons the block will take (83.5mm is what I'm seeing)?
Power comes from boost, not displacement. So I recommend the smallest pistons that will work for your block. Exact size is up to you.
I've been tearing my motor apart this winter as well. Mazda must've had a good laugh putting that oil drain (or whatever it is) in the block. I freaked out as well, I thought my engine was toast.
I've been tearing my motor apart this winter as well. Mazda must've had a good laugh putting that oil drain (or whatever it is) in the block. I freaked out as well, I thought my engine was toast.
Haha. Was a brutal joke but I'm glad the block is okay. Good luck with your build!
Heeding Emilio's (and others) advice I'll forego the forged oversized pistons for either standard or +0.01" oversized OEM pistons. This'll leave more meat on the bone for future rebuilds which is nice. Called a few machine shops today to try to narrow down where to get this honing work done. I think I found a good option so I'm excited about that. Only downside is he doesn't have a torque plate but no one I called today did... so, ohh well. Maybe for the next build. Anyway, I ordered the ARP head & main studs from 949 so as soon as they come in, I'll go ahead and drop off the block with the main caps installed. In the meantime I'll clean the block/head as best I can. Machine shop will let me know which bearings and pistons will be appropriate. I also got what I need to do some porting on the head, but a bit more research to be done before I start that...
Well. I had my friend (the one I borrowed the bore gauge and micrometer set from) over to double check my measurements. Looks like my zero wasn't perfect, so all the bores are actually within (or even smaller) than spec. So I'm hopeful a bore/hone to the first oversize will be sufficient to get a fresh cylinder walls.
ARP hardware came in last week!
Over the weekend I got the block and head cleaned up and ready for the machine shop.
I'll be dropping both off at the machine shop today. I don't think the head needs to be decked (I put a straight edge on it and no light passes through), but I figured I might as well have them take a look at it to make sure I'm not missing anything.
Some mistakes were made, though. While removing the valves I accidentally nicked two of the cam supports (cam journals? cam bearings? not sure what they would be called). If I knock down the edges can I still use this head or is it no good? I tried running my finger and cloth on it to see if it would catch an edge but the material is not protruding towards the cam.
Last edited by HalalBuilt; 01-15-2024 at 09:55 AM.
Dropped the block and head off at the machine shop last week. He was confident we could get clean cylinder walls without an aggressive overbore. So I went ahead and ordered 0.010" oversized OEM pistons and the applicable OEM rings. Also ordered Manley forged rods along with all the ACL race bearings and an OE gasket set.
Got the last of the packages last night! 949 packages not pictured. So I'm dropping the crank, pistons, and bearings off today. Not sure if he'll need the rods, but I brought them with me. While waiting for parts I asked him to check the head so I can pick it up when I came to drop this stuff off. He said it was a bit twisted and he had the take 0.0035" off to get it flat. That's not enough to be worried about volume change, right? Or should I check it?
Beefy boi rod. I'm actually shocked at how light the OEM pistons are. I guess I was expecting them to be heavier.
If all goes well I might be able to start rebuilding in a week or two!
Looking good! What is the cost difference between forged vs cast pistons? I would've thought it'd make sense to just get forged pistons if you're getting new pistons anyway but I also see plenty of "rod only" builds on here.
Looking good! What is the cost difference between forged vs cast pistons? I would've thought it'd make sense to just get forged pistons if you're getting new pistons anyway but I also see plenty of "rod only" builds on here.
Thanks! Didn't make the decision to go cast for cost savings. It's actually not as crazy of a difference all things considered.
Oversized OEM cast pistons: $312 Oversized OEM rings: $166. OEM total: $478
Forged oversized pistons (Supertech): $570 Rings (Wiseco): $134 Forged total: $704
$225 difference between the two. But to go with forged pistons I would need to skip over to at least a 0.5mm overbore. Not worth it IMO. The cylinder walls were still in good shape anyway.
Thanks! Didn't make the decision to go cast for cost savings. It's actually not as crazy of a difference all things considered.
Oversized OEM cast pistons: $312 Oversized OEM rings: $166. OEM total: $478
Forged oversized pistons (Supertech): $570 Rings (Wiseco): $134 Forged total: $704
$225 difference between the two. But to go with forged pistons I would need to skip over to at least a 0.5mm overbore. Not worth it IMO. The cylinder walls were still in good shape anyway.
Gotcha.. that sucks they don't make forged pistons less than .5mm over. Seems a little odd on a platform as popular as the Miata but who knows. I didn't think the wall thickness would be that big an issue but reading Emilio's post above clearly it does..
If you're using stock cast pistons you're not going to get to the power levels where cylinder wall integrity becomes a problem anyways. You opted for a much weaker piston to get 83.225mm bore size if I'm reading the specs correctly, when you could have done 83.5mm with forged? The difference of 0.275mm bore size is not going to suddenly make it way more resilient, but using forged instead of the cast pistons definitely makes a difference in the 300whp+ range. Plenty of people running 83.5mm or 84mm bore sizes are at 400+whp, but stock cast pistons would be the weak link at that point.
Straight from a rough engine building guide from Emilio:
Bore size - OEM is 83.0mm. The max for N/A is 85.5mm which requires a custom head gasket as the OEM just barely clears a 85.0. Low boost F/I ok up to 85.0mm. F/I high boost, we like to stick to 84.0mm to retain maximum cylinder wall thickness for bore stability. Anything 85.00 or bigger should get the block sonic tested. It is up to your pro engine builder to decide how much is enough. Power from bore increase is (generally) linearly proportional to bore delta. IOW, 83.00 to 85.00 is about 2.4% so a 150whp build becomes a 153.6whp build, or thereabouts. Personally, I love a 10.3:1 85.5 BP6D N/A build with a good flowing head for a 2100# HPDE build. Kinda of a sweet spot.
12-31-2023, 10:10 AM
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