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Thread: Gibtec piston/wall clearance
08-31-2020, 04:30 PM #16
I have been so interested in the thread I completely forgot about forum etiquette! My apologies Tbird, I had no intention of creating any hint of thread drift! I think this might be related but I don't know if it should continue here or in a new thread. If a new thread is suggested, you or Ed start it and let me know the the title. Thank you all for your patience!
I am reviewing Ed's latest post now and will reply back here later unless a suggestion is to move elsewhere.
08-31-2020, 05:21 PM #17
I wouldn't worry too much, Kirk. Your digression was a more or less natural expansion of the topic as you dug into related engine building considerations. Tbird is pretty good about those things and so am I. We just try to avoid threads spiraling into the ether on unrelated topics. Your stuff is right in the thick of the engine building experience and valuable to anyone that is building or preparing to begin their build.
All good, no harm and no foul — actually a lot of good.
09-01-2020, 06:55 PM #18
Ed, I'm about to buy some of the 9mm pins from Cobra Engineering. But in a past post I think you referred to the pins as "my" pins. Are both you and Cobra selling them or do you just collaborate with him? I want to support our vendors and this site but need a little direction. Same deal with the pistons. Deal direct with Gibtec or . . . ??
09-01-2020, 07:20 PM #19
The reason I selected the 4.6 was primarily the smaller bore size to get a DOHC V-8 motor to fit a specific class size. The old 262 (not 265) SBC has a smallish bore too but of course 2 valve Neanderthal pushrod technology. So the issue is the 0.093” thick Modular integral cylinder liner for a blown alcohol application. It doesn’t help that all the Teksid blocks have many miles on them and need a clean-up bore. Thanks to this forum I have learned that Gibtec pistons are not restricted to the traditional .020 or .030 overbore but can be ordered in any size 0.001” at a time. That is great because I can now size only what is necessary for cleanup and no more.
However, the .093” wall bothers me a lot. I was going to sleeve the whole block and put in stock bore, 0.125” thick sleeves. In talking to a machine shop he advised that it was a bad idea and boring out the liners as well as additional aluminum (due to the increased wall thickness of the sleeve) would drastically weaken the whole block. He had seen a cut up 4.6 and said something about how much material was left down there near the mains. I was left with the impression that thicker, stronger sleeves would be useless due to drastically weakened the block.
Your wisdom and comments are valued.
09-01-2020, 07:43 PM #20
There is some truth to the reduction in liner support aluminum when you go to the replaceable liners. The small loss of material turns out to be inconsequential. there are a number of guys who have gone to the replaceable liners on the Modmotor and a huge number who have gone that route on the Coyote, even with its larger bores. The 0.125" wall liner is where you want to be for maximum effort engines.
In my Forum Article Why You Want to Use Standard Bore Sizes you will see a block that has been modified to accept the replaceable liners and you will also see the liners from the bottom (crankcase side) and the top (block deck) side. The flanges on top have been cut to provide receiver grooves for stainless wire o-rings and dead soft copper head gaskets like the nitro engines use. At about 30 psi of boost an MLS gasket is pretty much done. Above 30 psi of boost you are foolish to run MLS gaskets. You need dead soft copper and stainless wire o-rings.
BTW those pistons in the pics are the ùber high compression Gibtec's I spoke of. We had to make a combustion chamber mold for Gibtec to scan, that I commented to you about, to create them.
Last edited by eschaider; 09-06-2020 at 02:51 AM. Reason: Spelling and Grammar
09-02-2020, 11:01 AM #21
Apologies for not responding sooner, got distracted.
The pins are a Cobra Engineering piece, and should be ordered directly from Cobra Engineering. The pistons are a Gibtec piece and should be ordered directly from Gibtec. When you contact Gibtec ask for Nick Plantus who led the internal design effort on the pistons. When you reach him tell him you need a set of the pistons he and Ed Schaider developed for the Modmotor. Buy 10 pistons so you have two spares.
In both instances I am a collaborator but not involved in the actual business of producing or selling the parts. When you get the chain tensioner pivot pins be sure to get the drill fixture for installing them. If you do not, you will need to use a machine shop to drill and tap the holes for you and the fixture will look inexpensive by comparison. Additionally this will not be the last Modmotor you build especially for racing so the fixture is a good tool investment.
Last edited by eschaider; 09-15-2020 at 10:59 AM. Reason: Spelling and Grammar