.100 OS Pistons

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ModelTWoods
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* REQUIRED* Type and Year of Model Ts owned: 1927 Model T coupe, 1926 4 door sedan
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.100 OS Pistons

Post by ModelTWoods » Sun Feb 22, 2026 12:42 pm

I see Mark Chaffin has a set of .100 OS pistons for sale on the Classifieds. Has anyone ever bored a T block that much and is there any danger or chance of failure due to boring a cylinder wall that much? I'm not an interested buyer but just wondering how much a T cylinder wall could be bored and still retain a reliable motor. Could a regular head gasket be used with a bore that big?


speedytinc
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Re: .100 OS Pistons

Post by speedytinc » Sun Feb 22, 2026 1:40 pm

Feeling lucky?
You wont know until the holes are bored.
Between core shifts & corroded water jackets, the outcome is not assured.
But, if you do go thru, your bores are closer to being sized for new sleeves.


Erik Barrett
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Re: .100 OS Pistons

Post by Erik Barrett » Sun Feb 22, 2026 1:57 pm

The Wild Cherry 2 race engine is bored .137” oversized. Same bore as a standard model
A. We have done this to two blocks. The only problem we have had is keeping the head gasket between the front and rear pairs of cylinders. The margin between them is only about a quarter inch. We have to machine the deck for a steel
O-ring and use a solid copper head gasket. Lots of other things break, but we have never had a cylinder blow out.


speedytinc
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Re: .100 OS Pistons

Post by speedytinc » Sun Feb 22, 2026 2:26 pm

I have heard of model A's being bored .125 over successfully.
On the other hand, I had a .030 over T come in with a water jacket leak.
Depends..........

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RajoRacer
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Re: .100 OS Pistons

Post by RajoRacer » Sun Feb 22, 2026 3:08 pm

Had a customer do .100 over on the block - it was O.K. but the pistons hit the cylinder head in several spots !

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