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Rules say “largest OEM piston not to exceed 0.20”. Where do you find these 0.20 pistons? Pelican has only 0.25 and 0.50.
Should I be replacing my connecting rods when I replace the pistons? Or maybe con rods are considered especially durable and generally not replaced.
Ranger wrote:
[quote]Rules say “largest OEM piston not to exceed 0.20”. Where do you find these 0.20 pistons? Pelican has only 0.25 and 0.50.
Should I be replacing my connecting rods when I replace the pistons? Or maybe con rods are considered especially durable and generally not replaced.[/quote]
“not to exceed .020 inch.”
.020 inch is ~ 0.50 mm
.010 inch is ~ 0.25 mm
pelican is using metric sizing…
Didn’t IndyJim recently post a smoking deal on these at BimmerWorld?
Does increased piston size affect compression ratio? I assume it must since the volumetric difference between stock and 0.020"-over pistons doesn’t seem like a good value ($500-1,000 per hp?).
Steve D.
PS - I also assume used con rods are OK provided you find/create a set of 6 matched-weight sets of piston, wrist pin, con rod assembly.
My perception is that our pistons have durability problems because they are cast, not forged. So the grooves loose their shape and don’t properly position the rings. So when you rebuild a high mileage motor, you should replace the pistons.
The machine shop says that the cylinders in my spare block are a little out of round but not critically so. Therefore I could replace my 180k mi. pistons with standard or over-bore. And since there isn’t much cost difference between the two piston sizes, I figure that I might as well go overbore.
Besides, “going overboard” on something is kind of my natural state anyhow.
I have to decide about my con rods tho. I saw some outfits that “recondition” them, but it’s pricey.
As usual, I sound like I know what I’m talking about, but all of this is just a product of a couple night’s research. I don’t know shit about rebuilding a bottom end.
Ranger wrote:
Your honesty and sense of adventure are admirable.
You mean, like, removing all that pesky weight they have accumulated over the years? 
Since it’s just you and I talking in private, I will tell you that virtually all of the gains in “pro” spec miata motors come from the head (valves, cam profiles (until the new spec came out last year), decking the head to increase compression). Other than good bearings, nobody does much to the bottom end (other than balancing/blueprinting, which is illegal in SM rules but legal in ours).
With non-adjustable FPR’s and stock airbox, which component is the limiting factor? Do we know if the motor can suck enough air and fuel in to take advantage of the apparent mathematical advantage of an overbore?
Steve D.
dmwhite wrote:
[quote]Ranger wrote:
[quote]Rules say “largest OEM piston not to exceed 0.20”. Where do you find these 0.20 pistons? Pelican has only 0.25 and 0.50.
Should I be replacing my connecting rods when I replace the pistons? Or maybe con rods are considered especially durable and generally not replaced.[/quote]
“not to exceed .020 inch.”
.020 inch is ~ 0.50 mm
.010 inch is ~ 0.25 mm
pelican is using metric sizing…[/quote]
Thanks Dave, that was the answer.
And yes Bimmerworld has a smoking deal on this. If you’re going to go ‘overbored’ I honestly don’t think you’ll find a better price on pistons.
Correct me if I am wrong, which is frequently, if you are going with over bore pistons don’t you have to overbore the cylinders?
So if Rangers block is slightly out of round and they re-hone the block, you would have to do the overbore pistons because of the material removed to make the cylinders round again correct?
You would be able to use stock pistons if no honing of the cylinders is needed right?
BeerTech >>> parts replacers…NOT mechanics.
Steve D wrote:
[quote][Do we know if the motor can suck enough air and fuel in to take advantage of the apparent mathematical advantage of an overbore?
Steve D.[/quote]
Well given the same head, larger pistons, freshly honed overbore, and fresh rings should suck in more air than a 200K junkyard engine.
Don
Use the smallest overbore possible. Give yourself room later. Have the rods reconditioned…any good engine machine shop can do this. Have the crank journals polished…and clearanced for the new bearings. If you don’t have the equipment/knowledge to do this, have it done…it is critical to the life of your motor. Then have a second party check the first…period.
Have the block dipped…but be sure to have the shop remove the intermediate shaft bearings. Remove the plugs from the oil gallery…reseal with 3/8" pipe plugs…requires drilling and tapping after running a gun cleaning brass brush through all the galleries. Pressure wash at least 30 minutes and scramble to get dry and oiled before rust forms.
Those of you who were wondering, the cost for the above in in the neighborhood of 600-700 dollars…labor. And you wonder why a motor cost so much to have rebuilt. And that is just the block. Time spent on the head is actually more hours because of all the labor involved with the valves. Chuck
Understood, Don. But there are 3 factors that affect the energy potential of the “boom” in the cylinder - volume of air, volume of fuel, and volume of combustion chamber. if the planets line up, those 3 are equally and optimally matched. More likely, one of the 3 is your limiting factor. With the 1.6 liter Miata engine, it is air. Even if you stick 1.8l injectors on it, it can’t pull enough air through the afm to take advantage of the additional fuel. At least that’s what I was told when we found the prior owner had installed those on my car. After my friend put it on the podium at VIR. 
The overbore is only going to help to the extent you can deliver more fuel and air.
Of course, it is not my money so I don’t mind if Ranger experiments…
Steve D.
Jones wrote:
[quote]Correct me if I am wrong, which is frequently, if you are going with over bore pistons don’t you have to overbore the cylinders?
So if Rangers block is slightly out of round and they re-hone the block, you would have to do the overbore pistons because of the material removed to make the cylinders round again correct?
You would be able to use stock pistons if no honing of the cylinders is needed right?
[/quote]
Yes, I could put stock pistons in. But they’d have to be new stock pistons because old cast pistons inevitably have worn ring grooves. And I’d have to suck up the fact that the slightly egg shaped cylinders impact cylinder sealing.
The cost of the larger pistons and overbore is ~ $300 total. Easy call.
Scott Gress & Al Kritter —> BeerTech South.
cwbaader wrote:
[quote]Use the smallest overbore possible. Give yourself room later. Have the rods reconditioned…any good engine machine shop can do this. Have the crank journals polished…and clearanced for the new bearings. If you don’t have the equipment/knowledge to do this, have it done…it is critical to the life of your motor. Then have a second party check the first…period.
Have the block dipped…but be sure to have the shop remove the intermediate shaft bearings. Remove the plugs from the oil gallery…reseal with 3/8" pipe plugs…requires drilling and tapping after running a gun cleaning brass brush through all the galleries. Pressure wash at least 30 minutes and scramble to get dry and oiled before rust forms.
Those of you who were wondering, the cost for the above in in the neighborhood of 600-700 dollars…labor. And you wonder why a motor cost so much to have rebuilt. And that is just the block. Time spent on the head is actually more hours because of all the labor involved with the valves. Chuck[/quote]
I’ll take your post to my machine shop, thx.
What is the purpose of removing the oil galley plugs and then putting in the pipe plugs?
I was told today “whatever you do don’t remove the intermediate shaft bearings because they can’t be reinstalled. If the bearings are removed the block becomes junk.” Your thoughts on this?
Ranger wrote:
I hate to harsh your buzz with logic and math, but the cheapest 20 over pistons are $299 (from BimmerWorld)… I’m all for trying things myself, but boring the block for $1 in your garage isn’t really a good idea.
Ranger wrote:
If that’s the case… Which one of you two is selling your car? And one of you needs to start wrecking your car much more frequently. And run underweight all the time and then get DQ’ed.
TheRedBaron wrote:
[quote]Ranger wrote:
I hate to harsh your buzz with logic and math, but the cheapest 20 over pistons are $299 (from BimmerWorld)… I’m all for trying things myself, but boring the block for $1 in your garage isn’t really a good idea.
Ranger wrote:
If that’s the case… Which one of you two is selling your car? And one of you needs to start wrecking your car much more frequently. And run underweight all the time and then get DQ’ed.[/quote]
Who wrote this? Is it the guy who used to race the #82 with us? I think I remember him. I heard he is a full time cat nanny now who has an abusive spouse. Let me guess… the bruising on your face and nasal area are from falling down the steps right? I know, I know, you just make her crazy, she hits you because she loves you Craig.
TheRedBaron wrote:
[quote]Ranger wrote:
I hate to harsh your buzz with logic and math, but the cheapest 20 over pistons are $299 (from BimmerWorld)… I’m all for trying things myself, but boring the block for $1 in your garage isn’t really a good idea.
Ranger wrote:
If that’s the case… Which one of you two is selling your car? And one of you needs to start wrecking your car much more frequently. And run underweight all the time and then get DQ’ed.[/quote]
Over bore pistons are about $100 more then standard pistons. Overbore of block is < $200. So overbore is $300 more costly then standard.
Al crunched JP’s car last month at CMP turn 8. I bruised mine at the same place last weekend. I’d say we’re on track.
My 400qt Accusump and aux oil pumps are going to make it a challenge to be underweight.
Jones wrote:
Speaking of abuse Jones, you’re going to get abused so bad at Barber you’re gonna be calling the cops like the white trash trailer park mistress that you are.
The stifling heat of the jet black paddock and the constant pestering of the parking lot police will take the p!ss and vinegar out of you two. At least you’ll appreciate the landscaping.