High compression head problems

trainguy

trainguy

Well-known member
A friend is contimplating installing a high compression head on his stock 1926 engined 14 rdsr . He is concerned about the longevity of the crank and Babbitt main bearings. Do any fellow Barners have any comments on the matter ? thanks Phil
 
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High compression heads do not harm the Babbitt if the bearings are in good condition and clearances are correct. The spark advance with a high compression head is more critical in that you will not need as much advance. You will experience spark knock if you advance the spark too much. Too much spark advance may cause you to join the two piece crank club. The nice thing about the high compression heads is the ability to overcome hills with ease. Hope this helps.
 
If the bottom end is good the 6:1 should be ok, if he is looking for higher that's a different story.
 
I've been on these Forums for a number of years and I don't recall any rash of problems like the ones you mentioned. At least no ones connected high comp heads with broken cranks or bearings. I guess 29s has a high comp head car so I'm assuming the timing business is true.
 
He was thinking 8to1 compression, which I feel is a bit much and most likely lead to problems. Phil
 
The Prus head is advertised at 8:1 but as it turns out it is more like 6:1. Lots of guys using them, no reported problems. I think that there is a tendency for humans to imagine problems when there are none.

trainguy said:
He was thinking 8to1 compression, which I feel is a bit much and most likely lead to problems. Phil
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I put a Z head on a 1926 one time only to discovered the block had been decked causing the pistons to hit the head.
 
CharlieB said:
At least no ones connected high comp heads with broken cranks or bearings. I guess 29s has a high comp head car so I'm assuming the timing business is true.
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It's not the high compression head that causes cranks to fail. I have high compression heads on both of my T's. The wrong spark advance can cause the crank failure.
 
Apparently letting the engine warm up before going for a drive causes crank failure. I was letting the engine warm up on a 20 degree January day when the crank in my signature broke. Completely stock engine, pan was straight as an arrow.

You are fooling yourself if you think anything you did made the crank break. They will all break one day - you can't predict when. They were poorly made when new, of questionable materials, in a size that was just strong enough for the intended use. The Model T has a lot of great engineering, the crankshaft excepted.
 
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