Piston ring gap. Forged 1.6 79mm
 

Hello all,

I recently received my 1990 1.6 engine and parts back from the machine shop after we found that it was badly down on compression last fall. The work was done by Alderman Machine on Rt 13 here in DE if anyone needs a top notch shop in the area. While I await my rebuilt head I am slowly working on the bottom end. I want to run my piston ring gap options by the gurus here before I start using my filing tool on them.

As engine bore, boost level, and vehicle usage take effect on this number I am including some pertinent details below.

1990 Miata - weekend, road rally, track day
M45 Supercharger, non-intercooled, 7 psi pulley
Stock block - overbored to 79mm
1993 big nose crank
Wiseco forged pistons - K554M79
Wiseco piston rings - 7900XX
Eagle H Beam forged rods
King Tri-Metal Main/Rod bearings
Megasquirt MS2PNP
FF 640cc injectors/190lph fuel pump
Qmax coolant re-route
Fidanza lightweight 1.6 flywheel (7.6lbs)
195 degree stant thermostat (standard w/ Qmax)
Cometic .040 head gasket
ARP head studs
AC delete & Steering rack de-power by MiataCage
TDR SC pulleys w/ track setup
GM IAT sensor in crossover tube
Thermoswitch delete (fans controlled by J6 jumper in MS2)

The Wiseco sheet and some websites have left me with three gap options for the top and second rings. Bore at 79mm = 3.1102"
High Performance Street/Strip - .0045/.0055 = .01399/.01710
Street-Moderate Turbo/Nitrous - .0050/.0055 = .01551/.01710
"Rule of thumb" pulled from old posts - .0050/.0060 = .01551/.01866

After measuring the supplied rings in each bore at a 1 inch depth:
Cyl 1
1= .012
2= .018

Cyl 2
1= .013
2= .019

Cyl 3
1= .013
2= .020

Cyl 4
1= .012
2= .020

To me it appears my 2nd rings are about ready to go with Wiseco's tolerance of .004. I may open up #1 just slightly to .019. However, my top rings need to be opened up. With the above setup should I go up to .014, .015, or .016 to stay on the safe side? I appreciate any input you guys have.

Thanks!
Attachment 227079
Attachment 227080

.012-.013" is not enough. I recommend checking your piston to wall clearance before increasing the ring gap.

thats not good too be too tight(unless you are a girl ;) ;) ) on boosted cars.
the hea dgenerated by the higher than atmosphere boost levels will expand the rings so much they may lock up and scratch the cylnder walls.
go by the performance reocmmended by weiseco and keep an eye on the oil.
you will burn a bit, so don't worry about the smoke.

Thanks!

I pulled the sheet from the shop and found that the bores are set to 3.1110” or 79.0194mm. They have the piston measurement at 3.1085” or 78.965mm, leaving a wall clearance of +/-.0025” on each. The only thing they didn’t touch were the rings so filing them to size is up to me. I’ve opened up my top set to .016 across the four and have the second set at .019/.019/.020/.020 going from 1-4. Absolutely could not leave them as tight as they were without having expansion issues down the line.

3.1110” - 3.1085” = .0025". It will seize and blow up. Do not file your rings and take it back to the shop to have them re-hone it to at least .004" PTW. Remind them it is a forced induction application with a 2618 forged piston. The ring gaps will open up .003" per .001" increase in bore. You will be +.0045" after re-honing. Have them pull the oil restricter and galley plugs after honing to facilitate cleaning the oil passage's.

TBH, if the mahcine shop didn't bother asking him about his goals, maybe don't go back.