Another bearing clearance thread
 

EFR 6258 build. goal 350ish whp. Dual duty HPDE and street car. Not daily driven. Probably 3-5k miles a year on it with 5-6 track days.

Just got my block back from the machine shop and here are the clearances.

Mains
1 - .0018
2 - .0013
3 - .0014
4 - .0013
5 - .0013

Rod
1 - .0022
2- .0023
3- . 0023
4- .0021

Going to run ARP main studs and the block was align honed.

Happy with the rods, but a bit worried about the mains. I feel that is a bit tight, I would prefer them all to be around .0018 from what I have read around here, but I really do not know if this a huge issue. Willing to do what it takes to get this right before I assemble.

My thought is that I could maybe go with the ACL HX bearings which provide .001 more clearance and use only 1/2 shell from each (I believe this should give me an extra .0005) for mains 2/3/4/5, but leave standard bearings in main #1. That would be the following

1 - standard bearing - .0018
2 - 1/2 standard 1/2 HX - .0018
3 - 1/2 standard 1/2 HX - .0019
4 - 1/2 standard 1/2 HX - .0018
5 - 1/2 standard 1/2 HX - .0018

I believe this is ok to do, but not sure if having some oversize and some standards is normal practice (I know mixing shells is, but not sure if doing it some bearings but not others is)?

I also understand that if I do this I should put the "larger" shell on the block side (is that the larger clearance)?

Any thoughts out there?

EDIT:

Update for the plastigauge measurements I just took.

I plastigauged the cap side and the block side of the mains. They are all appear really really close to the same size clearances according to the plastigauge.

The plastigauge mark is narrower than the .0015 but larger than the .002 size. I would say they all appear to be right around .0017. According to the plastigauge I see no way they can be .0013, the squished gauge is for sure "thinner" than the .0015 measurement.

So with that new information, does anyone think this is too tight of main clearances? I am inclined to build it as is with standard main bearings up since I am confident it is now at .0017 clearance. That is just a tiny bit tighter than I would like, but I cant imagine that will make or break the motor?

Please correct me if I am way off here!

I agree the mains look a bit tight on a high HP build. I'd want them .0018 to .0025.
I'd talk with the machine shop that did the work.
Strange to see the first one is OK and the other are different.
I'd want a good explanation for this as the align honing should NOT create this.
Was the crank machined?

The crank was not machined. It was in great shape.

Here are the full measurements of the mains that I was provided with from the machine shop

Block
1 - 1.9720
2 - 1.9718
3 - 1.9715
4 - 1.9715
5 - 1.9719

Crank
1 - 1.9702
2 - 1.9705
3 - 1.9701
4 - 1.9702
5 - 1.9705


I know it isn't exactly as scientific, but I am going to check this all with plastigauge and report back.

You mean are running ARP main stud's since they needed to be installed for align honing. Something doesn't seem right about your numbers. I would double check your measurements. You can run half of a shell of the .025 bearings. Make sure you get the correct set since acl makes a + & - set. Has the crank already been polished? What rods are you using? A good target number for clearances is .002" +- .0002".

Most machine shops do not have the capability to do an actual measurement. The numbers do not mean much only the difference. Measuring the housing bore dims requires standards. Preferable standards that are at the min max dim. Most or no shops will have them for a miata eng.

LeoNA.

I had the bearings at the shop when they measured. They measured the diameter of the crank and the diameter of the main bores with new ACL standard bearings installed and the acl main studs torqued to 60 ftlbs. Those are the numbers they got from that.

Update for the plastigauge measurements I just took.

I plastigauged the cap side and the block side of the mains. They are all appear really really close to the same size clearances according to the plastigauge.

The plastigauge mark is narrower than the .0015 but larger than the .002 size. I would say they all appear to be right around .0017. According to the plastigauge I see no way they can be .0013, the squished gauge is for sure "thinner" than the .0015 measurement.

So with that new information, does anyone think this is too tight of main clearances? I am inclined to build it as is with standard main bearings up since I am confident it is now at .0017 clearance. That is just a tiny bit tighter than I would like, but I cant imagine that will make or break the motor?

Please correct me if I am way off here!

I prefer stock un-machined cranks for performance builds but...
I always have them "micro-polished".
My machine shop uses a specialized belt sander running a skinny abrasive belt on the journals while the crank is spinning on a crankshaft grinder.
It produces a super fine finish to the journal and REDUCES the diameter 2-3 tenths.
This provides the slight increase in bearing clearance I'm looking for in a higher stressed engine.
I usually have them balanced as well but this is not critical for your goals (especially on an inline engines).

If you are basing your decisions on their measurements you are doing it WRONG.
"Trust but Verify"
I just measured a 04 MSM un-damaged main journal at 49.97mm (NSK dial caliper) and 1.9673 (Starrett #436 micrometer) so your concern regarding their accuracy are well founded...

Check clearances yourself before posting what may be a non-issue...

You are WAY OFF.
Plastigauge is ONLY used on ONE side of a completely dry crank/bearing interface...
If you did it on both sides (at the same time) the readings are useless. If you checked both sides individually you are fine (but you did a bunch of un-necessary work).
The crank rests on the center of bottom shell and the plastigauge checks the total bearing clearance in one spot at top 180 degrees from the resting point..
If there is ANYTHING between the crank and the bearing shell the measurement is trash. Oil (bearing lube) will create in-accuracy.

The age of the plastigauge is paramount as well. If it could be older than 6 months I will not use it. I know this from watching unused plastigauge slowly shrink in the packaging.
By the time it's a year old it's 1/3 the diameter of new stuff. I purchase from Summit as I believe their stock is the newest available.
Getting the crushed plastigauge off of the crank and bearings can be tricky. I use fingernails only and remove the last remaining sheen with B12 carb spray.
Plastigauge is a final check for already measured parts in my book. If you rely solely on it you are asking for trouble...

One thing to be aware of is bearing crush. Each time you torque up a journal you crush the bearing a little tiny bit and if you do it enough times the bearing will be a bit loose in the bore.
This can lead to a "spun" bearing. During mock-ups for checking clearances I will back off the torques a bit to reduce this (60 will change to 40).
I try to do only 2 mock ups before final assembly.

Measure them again with the plastigage and report back. Use the arp lube ,on the threads, washer and nut. Make sure the parting line surfaces of the cap and block are clean. If the lube is dry or thickened you can mix in a few drops of engine oil.

If I had a crank grinder or knew of a good shop with the correct width wheels for these engines I would never run a used crank again. Crank grinding does not weaken the crank and improves the straightness and journal size/roundness. The cranks from mazda are some of the worst I have ever seen. They look like they were ground by drunk rejects from other manufactures.

Plastigauge was done on completely dry and clean bearings/crank. One side done at a time. I did it twice to double check measurements two ways (I wanted to make sure it duplicated the measurements).

Honestly not sure on the age of it, but I did just purchase it so I would assume it is not very old.

Both sides does not make sense. The weight of the crank alone would give you a false reading if you were putting it on the bottom shell. The factory max clearance is .0014". If your a little over that your probably ok. I would rather be at .0016 then .0026" on the mains. Timing belt and an oil pump that runs off the crank engine require a little less clearance.

If your bearings are a little loose, you know. If your bearings are too tight, everyone will know. At least that is what I have been told any time I asked the old hats.

Just want to follow up on this thread as I have had a PM in regards to it. I went with the block how it was and have a few years of abuse on it. I am running an EFR6258 at max boost all the time (EBC is 100% closed) - at altitude (Denver). Assuming I am around 350-400 WHP (No dyno tune, been tuning myslef and with some help remotely from folks). Running e85. Engine seems all good so far, so either I can not measure, or the clearances I had are "good enough" for a fairly high horsepower build.

I bet by the time it broke in it had plenty of clearance and you've got yourself a good motor. With my build I did the opposite and just had a trusted machine shop machine and assemble it. No stress and the motor has held up to all the abuse I can give it over the last 5 years.

You're almost certainly not making 350 whp at Denver altitude without corrections. I'm not surprised you had no issue. Factory clearances have proven to be fine for the power you're making. Poscat for the follow up though.

Good to know how it all turned out. Thanks for the update

You may be correct. I really don't know how much I am making. I do know that with 245's on 10" wheels I can not keep traction for shit and the car is pretty quick. I should go dyno it someday to see. If I do I will update this post with a sheet. Maybe 300 or so uncorrected? Hard to say, All I know is pulls pretty damn hard/beats a lot of 500-700 HP cars I have drag raced and I am assuming those are approximately 2x the weight, so I would assume somewhere around 300-350 uncorrected (not sure if those cars HP are corrected tho). Either way, it is too much power for the track and gets WAY too hot, so I detune it when tracking it anyways.