I purchased a 525sc with 330 hours and fresh top end. Spent the last month or so installing the motor in my scarab 23 SCS with a heat exchanger.

Set the timing at 13 degrees base and 35 full advance, went out for a test run on the lake. Got about 10 minutes out, temps were all good, everything felt normal. Opened it up and held at 5000 rpm for about 10 seconds. Temp spiked and overheated , coolant sprayed everywhere, stopped the boat and shut off, wouldn't turn over, hydro locked. Got towed in.

Ripped the motor right down to the bare block. Oil filter was a little sparkley but didn't find any metal at all in the pan, or anywhere in the motor. Main bearings and rod bearings all look good. Pistons look good. Cylinder walls have minor scuffs. Machine shop says I will have to bore the block and get new pistons. Both head gaskets were blown out.

I keep thinking of why this could happen.. I had 200L of 89 octane in the boat from last year so I added 3 jugs of octane booster( just cheap STP booster). I had a 20L can of AV gas(100 octane) I was planning on adding once I burnt 20l but I didn't make it that far. Do you think it could have been the fuel that caused the motor to detonate and blow the gaskets out? Or maybe I had the timing too advanced ?

The motor is at the machine shop now, they are going to magnaflux the block and heads to check for cracks, size the rods, polish crank, and bore the scuffs out and I will likely pay them to assemble the bottom end.

I would like some advice on what people do when they rebuild these things. I have heard that they like to be bumped up to 8.25 - 8.5:1 compression. Does this sacrifice longevity at all?

​​​​​​What are the best pistons to use?

What are the best rings?

What are the best head gaskets? ARP bolts?

I realize there are lots of forums on this and I am reading all of them, but I would like some up to date response and specific info.

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Sorry to hear this. Who did the first rebuild? The same guys that are starting on the 2nd?
Go to Alex Haxby's place. I know it will cost more but it will be done right.

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13 base sounds like a lot of timing.

Chris

Most BBC's are pretty happy at 34* total advance, also depends on where the total advance comes in. Most likely something just gave up, spring, valve seat or something. Machinist should be able to tell.

So you had last years 89 octane in it with cheap octane boost?? Thats something I would definitely NOT recommend with a 525sc.

As far as your rebuild question, 8.5:1 compression, AK racing alum heads and a 731 cam will put you in the 650 hp range.

I know.. bad mistake by me, I had my AV gas I was planning on adding as soon as I burned off 20L but i just had to do a WOT pull. Trust me I'm kicking myself in the ass for it now.

Alluminum heads aren't in the budget this year as I just bought the motor, does the roller cam do well with the 088 heads? Was wondering if I should change the cam while it's apart.

Also, what pistons will bump me up to 8.5:1?

35° old gas on a blown motor at wot is a recipe for disaster.

731 or 741 does nicely w/ 088 heads

You might want to think about your plumbing. Are you sure you got all the air out of the block ? It usually takes a few runs to get the coolant topped off.

Could have also been an issue. I didn't top it off. I'm going to drill a pin hole in my thermostat next time, and top up.. and run 94 octane..

If it were bad gas or too much timing the pistons tops would look sand blasted. Use cometic mls head gaskets and arp studs on rebuild. Then before running again rebuild the carb, suck out every ounce of old 89 octane gas and refill with fresh 93 octane. Also make sure your block temp is running cool (150ish) and set all in timing around 30 before putting the coals to it again.

BT