High compression engine failure
Thread Starter
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Road Warrior
Joined: Mar 2009
Posts: 1,485
Likes: 232
From: Ohio
I forgot to mention that I have 40,000 miles on this build. When it went bad I knew it right away. I was in Indiana headed for Minnesota and limped it as far as Rockford Illinois and then on to Madison Wisconsin before I got a U-Haul to come home. Always ran great and sounded good so I doubt anything was interfering with anything else.
Road Captain
Joined: Mar 2009
Posts: 689
Likes: 6
From: Glen Rose TX
the combination of the .030 HG and .040 shaving of the heads seems like an awful lot.
I tried to run the Bigboyz compression calculator but to many unknowns to me... The WAG I got was 206CCP so pretty darned high
You know what you have so have a go at it yourself and it would help others to post up your full build..
http://www.bigboyzheadporting.com/TwinCamComp.htm
I tried to run the Bigboyz compression calculator but to many unknowns to me... The WAG I got was 206CCP so pretty darned high
You know what you have so have a go at it yourself and it would help others to post up your full build..
http://www.bigboyzheadporting.com/TwinCamComp.htm
HDF Community Team
Joined: Jun 2006
Posts: 12,774
Likes: 2,606
From: san antonio
the combination of the .030 HG and .040 shaving of the heads seems like an awful lot.
I tried to run the Bigboyz compression calculator but to many unknowns to me... The WAG I got was 206CCP so pretty darned high
You know what you have so have a go at it yourself and it would help others to post up your full build..
http://www.bigboyzheadporting.com/TwinCamComp.htm
I tried to run the Bigboyz compression calculator but to many unknowns to me... The WAG I got was 206CCP so pretty darned high
You know what you have so have a go at it yourself and it would help others to post up your full build..
http://www.bigboyzheadporting.com/TwinCamComp.htm
Not a metallurgist but there appears to be something wrong with the surface treatment of the crank pin or possibly the bearing rollers. Having a metallurgist take a look would be a good idea.
Suggest that you mock up every stage of the assembly and record all date, i.e., deck height, squish, piston to valve clearance and valve to valve clearance. You really need a Trock tool to check valve to valve clearance so maybe a local shop has the tool and can do the check.
Thread Starter
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Road Warrior
Joined: Mar 2009
Posts: 1,485
Likes: 232
From: Ohio
Thanks for the replies. After it was pointed out to me that we aren't so much talking compression ratio, but maybe squish that is the problem.
I haven't yet measured anything but in my research I see that most people say the HD piston sits about 0.005" to 0.009" in the hole, and with a 0.045" head gasket, the stock squish should be around 0.050" to 0.054". By shaving the heads, If I am not mistaken, I have not messed with the squish, only my compression ratio. With this in mind, the Darkhorse connecting rods are 0.003" longer than stock reducing the squish by that same amount. Add this to the 0.015" thinner head gasket, I am probably around 0.030" to 0.035" squish band. Probably ok to start but I have a lot of carbon build up on the pistons and heads. I haven't yet measured it all up again but if the carbon is 0.005" thick both on the piston and on the head, my squish has been reduced to around 0.020". Does this sound like a reasonable direction to look for the cause of my failure?
It was my mistake to think Darkhorse was talking about compression ratio. He did say squish during our conversation but I was ignorant to this measurement. I am just now learning about squish. 5 years ago when I was researching how to increase my compression ratio, I never found anything about squish. Poor research I guess. Now I am paying the price for trying to learn this myself. Oh well. I bet I will know more this time around than I did last time when I am done.
I had the metallurgists at work test the crankpin and the bearings and everything was normal. They concluded it was not a material defect.
I haven't yet measured anything but in my research I see that most people say the HD piston sits about 0.005" to 0.009" in the hole, and with a 0.045" head gasket, the stock squish should be around 0.050" to 0.054". By shaving the heads, If I am not mistaken, I have not messed with the squish, only my compression ratio. With this in mind, the Darkhorse connecting rods are 0.003" longer than stock reducing the squish by that same amount. Add this to the 0.015" thinner head gasket, I am probably around 0.030" to 0.035" squish band. Probably ok to start but I have a lot of carbon build up on the pistons and heads. I haven't yet measured it all up again but if the carbon is 0.005" thick both on the piston and on the head, my squish has been reduced to around 0.020". Does this sound like a reasonable direction to look for the cause of my failure?
It was my mistake to think Darkhorse was talking about compression ratio. He did say squish during our conversation but I was ignorant to this measurement. I am just now learning about squish. 5 years ago when I was researching how to increase my compression ratio, I never found anything about squish. Poor research I guess. Now I am paying the price for trying to learn this myself. Oh well. I bet I will know more this time around than I did last time when I am done.
I had the metallurgists at work test the crankpin and the bearings and everything was normal. They concluded it was not a material defect.
Last edited by ohioflhs; May 13, 2015 at 11:26 AM.
HDF Community Team
Joined: Jun 2006
Posts: 12,774
Likes: 2,606
From: san antonio
Your squish analysis is correct and while .020" squish is not desireable, I am having trouble connecting squish to the crank pin/bearing problem. Has the metallurgist checked out the roller in the bearing? I would have to have another conversation with John for an explanation of how a tight squish band would result in the damage to the crank pin. I would also want John's explanation of the failure occurring at TDC.
I see no reason not to reassemble and run provided that you mock up and measure all clearances and addressing any anomalies that result from the measurement. If everything fits up properly, it should run without issues.
I see no reason not to reassemble and run provided that you mock up and measure all clearances and addressing any anomalies that result from the measurement. If everything fits up properly, it should run without issues.
Thread Starter
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Road Warrior
Joined: Mar 2009
Posts: 1,485
Likes: 232
From: Ohio
Thank you. I have found nothing else that seems to be the culprit. I will pay extra special attention to reassembly this time. The first time I had a reputable engine builder in the area do the work so I expected nothing would fail this early. This time I am doing it mostly myself and hope it lasts longer this time around. I just can not come up with anything else to suspect. Thanks again.
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Seasoned HDF Member
Joined: Mar 2010
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From: Poolville
you really do need to measure everything and plug the info into a compression ratio calculator. till you do that you will have no idea as to what is going on...and once you get that then you need to figure out you dynamic cr and go from there.