Author Topic: Cause of fretting on main caps?  (Read 819 times)

Offline Dunc the Funk

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Cause of fretting on main caps?
« on: February 07, 2020, 01:11:46 PM »
We only put 13 passes on our blown methanol BBC last season. I was surprised to find serious fretting had occurred on caps 2,3 & 4. 4 was loose in it's register.
As far as I know this did not happen last season when we probably did 20 odd passes. Is there anything I should think about with the tune that may have caused this?

Offline jeff/21

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Re: Cause of fretting on main caps?
« Reply #1 on: February 07, 2020, 03:57:41 PM »
cap walk,caused by weak caps and/or studs

dreracecar

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Re: Cause of fretting on main caps?
« Reply #2 on: February 07, 2020, 06:54:43 PM »
Just the studs or are there "ring dowels" around the studs to locate the caps?

Offline dusterdave173

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Re: Cause of fretting on main caps?
« Reply #3 on: February 08, 2020, 04:37:07 AM »
Tune up
When timing too high etc or tune up is off there can be detonation--pre ignition etc
That is your culprit--back timing off some  Find out where the tune up issue is
I worked in engine shop for decades--even cross bolted hemis's etc will do it if timing too advanced etc
With correct tune up that goes to a minimum
Engines can run great ET's etc but are killing themselves half way down the track and you can't hear it
Plugs and main caps tell the tale
I have always had a fascination with fast cars at the expense of more normal character development

Offline Dunc the Funk

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Re: Cause of fretting on main caps?
« Reply #4 on: February 08, 2020, 02:09:57 PM »
Tune has been all over the place as we started the season with a failing fuel pump. This got it down to the leanest it's ever been where the plating was off the plugs down to the first thread. Still, it ran 184mph on an 8.4 with an on/off/on/off launch. Then we got a new pump & it went crazy rich even though I changed the pill because I expected it to be rich.

Not had more than 34 degrees advance in it so far.

We have just had the block & caps machined, doweled & line bored by a very experienced machinist. Getting it back on Tuesday.

Offline gordon

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Re: Cause of fretting on main caps?
« Reply #5 on: February 17, 2020, 08:03:05 AM »
Spot on by duster Dave. Tune up allowed Detonation,too much timing and or not enough fuel. Even a little will fret the caps. If you have to run that close to detonation , try to find an old can of dutch boy white lead. Used to be sitting next to lathe for dead center work.  Put a small amount on mating surface then torque as usual. Takes just a small amount. I believe Gene Adams now uses a very pure form of lanolin as it is easier to find. The lube that is sold for use with thread go, no-go gauges is lab grade lanolin. See MSC catalog .

Offline Frontenginedragsters

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Re: Cause of fretting on main caps?
« Reply #6 on: February 22, 2020, 06:19:32 PM »
Like it has been already said:
 Lean, too much timing advance, too much engine load.
The crankshaft is being pushed counter-clockwise during detonation while the momentum is clockwise.
You need to richen up the fuel and back off the timing and sneak it back in a little at a time.
Get some help reading the plugs and pull the pan during the week and look at your bearings.
Sooner or later detonation will also crack the crankshaft.
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Offline Dunc the Funk

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Re: Cause of fretting on main caps?
« Reply #7 on: February 23, 2020, 09:19:42 AM »
I would say we only did one run where it was lean. This took off the plug plating much further than any other run we've done, as I said, down to the top of the first thread. Maybe that's when it did the damage. The motor was much hotter than usual after the run. Previously we'd not done more than about half way round the face. I recall the 'timing' bar on the plug ground was usually near or just down from the corner. We didn't realise how bad the pump was til we put a new one on & it's been so rich we have not coloured a plug since. We went from having less & less fuel in the oil to having a shed-load. We won't be running again until April at the earliest.