The MGA With An Attitude
PISTONS, Running Clearance, Twin Cam - TC-310D

On 2/10/2012, Alan.Whitfield wrote:
"I first got to know Peter Wood in 1972 after a seized engine twice during running in periods. On both occasions it occured around 70-mph steady speed having covered in access of 1000 miles steady with progressive use of the throttle and revs. I believed all the stories of twincam reliability were true and contacting Peter was the last move before selling the car. Peter's first reaction was the usual, timing, fuel, camshaft timing, all the basic settings correct? As I have worked with large rail diesel traction engines all my working career I knew that I had it right.
His next question was how many miles has it done? His reaction was that 1000 miles was not enough and running in should take up to 3000 miles. He explained that twincam (Hepolite 9.9s) pistons have solid skirts and therefore expand greater than the pushrod engine and need more running in of the bore to ensure lubrication. Having renewed the seized piston (No3 with rings trapped in grooves) I covered the 3000 miles and ran the car until 2002 without any further piston trouble, even towing a small caravan where sustained high revs were used climbing hills.
In 2002 the bottom end was rebuilt by Peter with a brand new crank, Hepolite 9.9 pistons, later type rods and fully balanced. After running in properly it has not been touched since. I am not conversant with the replacement pistons available today, but I believe they have a fourth ring. Patience is a good way forward to reliability.
P.S. I haver never holed a piston. -- Alan

To which Bill Spohn replied:
"Or more initial clearance, which has always been my approach. The factory clearances were set far too tight for long use unless you did a ridiculously long break in process. No one today should build a Twin Cam engine using the original tight clearances. Yes, if you use wider piston to wall clearance, your engine will wear out sooner, but at the rate these cars put on the miles, chances are you'll be long dead before the engine wears out".

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