Turbo Built FA20 Clearances
 

Looking into rebuilding an engine I have with a damaged crank; everything else appears to be in good shape.

Reviewing the factory service manual I noticed some tight tolerances for a factory motor:

For example piston ring end gap:
Maximum end gap
No. 1 0.20 to 0.25 mm (0.00787 to 0.00984 in.)
No. 2 0.60 to 0.70 mm (0.02362 to 0.02756 in.)
Oil ring 0.10 to 0.35 mm (0.00394 to 0.01378 in.)

Main Bearing Clearance
Standard clearance: 0.013 to 0.031 mm (0.00051 to 0.00122 in.)

Rod Bearing Clearance
Standard oil clearance: 0.025 to 0.055 mm (0.00098 to 0.00217 in.)

Looking at a couple aftermarket forged piston manufacturers I noticed depending on application there is quite a bit more gap being requested.

I did find that someone had posted an IAG Extreme Stage 3 block clearances which is a good reference point for example gaps.

Piston End Gap JE 86.25mm 3.396" 10.5:1
Top .020
2nd .023
Oil .015+

If I was guessing it looks like they used 3.396 * .0060 = .020376 for the top ring per JE blown race only recommendation.

However for the second ring 3.396 * .0063 = .0213948 isn't the recommendation for blown race only but closer to the minimum factory gap.

Main bearing clearance .0012 XPG STD

Rod bearing clearance .0018 ACL H STD

I thought it was interesting that their main bearing clearance is the loosest OEM and the rod bearing clearance was within factory spec also but toward the looser side as well.

Anyone have experience with the amount of blow-by that could be expected with this much end gap on the piston rings? Any builders out there with example clearances that would be a good rule of thumb or any additional modifications that should be done to support the motor?

:thumbsup:

edit:
DarkPira7e was able to get a copy of his IAG Stage 1 clearances:
https://www.ft86club.com/forums/atta...1&d=1609424558

I conclude that they aim for the loose side of main bearing clearance, rod bearing clearance is *slightly* loose, and the end gap is identical on the piston rings.

Thanks for the help guys, hopefully this information helps someone out in the future.

You should send an e-mail to IAG to see what their suggestions are. They've been great at answering a few I've had in the past.

The best advice I can give is to expect blowby, not to try calculate it. Catch it using catch cans and don't give it a chance to make it back into the system. Almost every EJ or FA engine I've seen get built without a catch can comes back with issues and gets a catch can.

+1 on the blow by and catch can suggested above

You will have blow by with pretty much any forged piston regardless of gaps as is the nature of them, so I would just go with manufacturers specs

I recently built my own engine, I had the machine shop gap the piston rings to JE spec, I have a little blow by on startup but not too bad and my catch can setup is easily dealing with it.

For the rod and mains clearance I went with OEM tolerances using King XPG, I was aiming for 300whp (supercharged) and wanted to do as much as possible to maintain oil pressure.

There is always a tendency to increase bearing clearances as power increases but many believe a smaller gap actually help maintain a better oil film/ dynamic wedge and it also results in a the better bearing loading, of course that varies depending on a number of factors. Just food for thought I guess

Has anyone used these in their build. We are building a fa20. New crank, king bearings, eagle rods, je 10:6.1 pistons. Based on their ring gap it said to do after the math top ring gap at .17 and the second at 19. Doesn’t seem right so look at this makes more sense just want to know anyone used it?