I'm in the process of building my first engine. I've got everything back from the machine shop and was starting to install my rods to my pistons when I realized the wrist pins don't fit through the rods.
I tried 2 rods and 2 wrist pins, but even with a decent amount of pressure from my hand they wouldn't slide in.

I've got Flyin Miata pistons and Eagle Rods, using the wrist pins supplied with the FM pistons. I don't know the specs on the FM wrist pins unfortunately as I failed to get the spec sheet back from the machine shop.

From the threads I could find on the topic, it seems that honing the small end of the rod isn't unheard of. However I've never seen anyone mention it in any of my research until now, and neither had my roommate, who has done 2 rebuilds.
I'll be reaching out to the suppliers of the parts as well, but was hoping to get a sanity check on here. I'd like to avoid another trip to the machine shop if I can, but I'd rather do it properly than twice.
The only other path I see is to return/exchange the rods, although that's not very appealing to me either.

I appreciate any feedback. I'm itching to get the engine back together.

 

Typically the wrist pins come with the pistons, and I would expect FM to obviously disclose if you needed specific rods. Most rods I’ve ever used are meant to work with OE wrist pins, and vise versa with aftermarket rods.

make sure you’re assembling with lubrication, but first and foremost, both pieces should be measured. Bring them into the machine shop, asks graciously if they could mic the pin and rod, to see if they’re the same (pin should be .001-.002 smaller), my guess is it’s just a tight fit on brand new parts.

 

Thanks Curly,

​​​​​A Friend of mine has loaned me his mic and T gauges which I'm getting more comfortable with. I'll check them tomorrow.

I tried to measure with a caliper and got different measurements but I realized that's not going to be accurate enough.

 

On my last build, I needed to have the machine shop enlarge the rod ends, in order to fit the piston pins.
I figured this was normal, and the machine shop certainly didn't question it. The rods were a different brand
to the pistons.

 

Yes, not a big deal. Actually, better this way as they can be honed to size. IF they were loose, nothing you can do. But tight, they are just honed to size.

I've built motors with rods as-is, but my last motor they were honed to size.

 

Got the measurements just now. All the pins measure in at 19.990 19.995mm (19.987-19.993 spec) and all the bores were 19.995mm 19.990mm (20.003 - 20.014mm spec). EDIT : Swapped the measurements around
Eagle specs the bores at 0.787 inches, or about 19.9898 mm.
Not sure if that warrants a trip to the machine shop, I'm debating on doing a scotch Brite hone.
I guess I assumed that the bores would be withing factory spec since these are Miata specific rods.



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So pins were .7868" to .7871", and rods were .7875" to .7879", making the smallest clearance .0004", and the largest .0011". If you're confident in your measurements, I'd move the pins/rods around so they're all around the same clearance and assemble them with lube. AFAIK, you want around .001", and that's right where you are. Use a small dead blow or rubber mallet to tap them in, they just need to go in perfectly straight.

 

Curly,

I probably didn't present that data clearly.

In short, all the bores measured 0.005mm smaller than the pins and 0.001 0.01 mm smaller than spec. EDIT : Apparently I can't make an error free post when I'm on my phone.

 

You could probably hone them a bit (a little at a time) with the ole dingleberry brush until you get tolerances where you want them, if you are feeling adventurous. It's pretty commonly done that way from what I've read.

I took mine to a machine shop though, they had to size them to fit my OE wristpins, like others have said they purposely undersize the rod small ends so that you can bore them to size.

 

I appreciate the input. That's what I'm leaning towards right now, but I need to do a bit of research first.

 

Curly, I had the measurements swapped in the post. Just edited it.

 

Ok, neither of us are crazy then. Don't do a scotch brite hone, that's a bad idea. You need a very specific clearance for the wrist pins and it's the last thing you want to guess at. Bring them all to the machine shop and have them hone them properly.

 

Yeah, after a tiny bit of research I think I'm going to skip the scotch brite idea. I'm headed out of town tomorrow so I think I'll drop the rods off at the machine shop before I skip town.

I went to install my crankshaft today as I thought about what to do about the rods. Looks like my main bearings are too tight as well, so I've got more to think about there. Having said that, I think it will be an issue for another day (and maybe another thread).

 

I'm under the vague impression that there are some rods where the wrist pins are expected to be a press fit, so that they will turn in the piston, but not in the rod. I have no idea if this is true of Eagle ones or not.

--Ian

 

According to Eagle's website these should be bushed for a full floating wrist pin. I though I saw something about them being bushed for press fit or full floating, but I must've been looking at the wrong thing as now I'm only finding them spec'd for floating.

Eagle's website if you're interested https://eaglerod.com/item/5523879-43...-miata-16-18-4

 

I got the small end my Eagle rods honed. Cost $6 a rod at the machine shop and turned it around in a day, was cheaper than buying the tool.

 

I reached out to Enjuku racing (who I bought the rods from) around the time I started this thread. Just wanted to give them props for great customer support. They played middle man between me and Eagle which go this response from Eagle,
"Based on the specifications given the issue is stacked tolerances where the rod is on the small side of the spec and the pin is on the high side.
The easiest solution would be to take a little out of the rod pin which would open up the rod to allow the pin to fit. Neither part is wrong it is where the parts are falling in regards to the required specification."

So pretty much the same response as I got here. The rods and pins are at the machine shop as of a couple hours ago, so I'm hoping to get them back before the weekend so I can keep making progress on the bottom end.

Thanks for the responses everyone.

 

Following up. Just got the rods back from the machine shop. They seemed a bit surprised that the rods needed to be honed. Cost $15 a rod, so I'm out $60.

Two ways of looking at this from my perspective.
1. I could've spent $60 more buying nicer rods, although it seems like a toss up if they would have the same issue as these.
2. I spent $60 getting a better fit on these rods than rods that I didn't have honed.

Regardless, I can get my pistons installed now, so I'm going to move on with the build.

 

I would personally ALWAYS expect new rod bushings needing to be honed. That IMO should be SOP...

And if not, at least should have been checked...

But I have learned that things I think should be standard SOP at a shop sometimes arent...