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Down in a hole
https://www.slantsix.org/forum/viewtopic.php?t=61158		 Page 2 of 2
Author:  bob fisher [ Mon May 29, 2017 5:38 am ]
Post subject:  are we approaching interference engine territory
hi slant sages- if you have zero block deck height, do you have an interference engine, where should the timing chain jump or bust the valves crash the pistons wasting the engine? understand the stock 225 and 198 are not interference but have heard it both ways on the 170 which some have insisted are interference. remember during the war seeing the head off a 170 block and those pistons were mighty close tothe top of the deck. however never saw a 170 crashed due to piston and valves colliding even with a busted chain. so whats the bottom line. thanks for the quick kindergarten lesson. ps have seen a number of recent ford 4cyl. engines wrecked this way when the timing belt went stinko. thanks tons bob f
Author:  Charrlie_S [ Mon May 29, 2017 7:37 am ]
Post subject: 
The 170 is an interference engine. If the piston is at zero deck (factory spec), and the head has not been milled (remember factory tolerance, here) the valves hit the piston at approx .150 lift.
Author:  bob fisher [ Wed May 31, 2017 9:47 am ]
Post subject: 
hi charlie- thanks for your reply on the 170. so if the chain jumps or breaks, , the valves will hit the piston and the engine will need big repair, correct? now if in fact the head was milled, the open valves would stick further into the cylinder increasing the crash damage when the chain broke or jumped , correct? sorry to be a pain in the keaster but one more question. on a stock 170 is it feasible to shorten the valve opening to eliminate the crash with the piston? guessing this would lead to the valve itself getting hotter(not good). thanks tons bob f
Author:  Charrlie_S [ Wed May 31, 2017 9:58 am ]
Post subject: 
It is not a question of shortening the valve event. If the chain breaks/jumps, the valve timing is moot. If the valve is open more then approx .150 at that point, the piston will hit the valve as it reaches TDC. The timing event must be taken into consideration, when the cam is purchased, and installed. Sometimes just changing the installed cam center line, will make a difference on whether the valve and piston play nice together, or not.

PS: On one of my early slant six builds, When I took the engine apart to freshen it up, I found that the valves had "self clearenced". All the pistons had very tiny valve reliefs.
Author:  Charrlie_S [ Wed May 31, 2017 2:15 pm ]
Post subject: 
I want to preface my interference engine info.
A stock 170 is an interference engine.
A stock 225 is not, but once you cut the deck, and/or head, and/or change the lift on the cam, it might be, depending on the amount.
Author:  CNC-Dude [ Wed May 31, 2017 2:34 pm ]
Post subject: 
At just 1000 RPM on an interference motor, the engine makes over 16-1/2 revolutions per second. If the chain breaks or jumps timing, the damage will be over and done before you could even react.
Author:  Dart270 [ Thu Jun 01, 2017 3:59 am ]
Post subject: 
Most engines in production are interference engines. There is no reason to fear this unless you have a really old timing chain. If you are worried, then replace the chain.

Charlie is dead right on everything.

Best,

Lou
Author:  Charrlie_S [ Thu Jun 01, 2017 6:23 am ]
Post subject: 
I would like to add one other thought. If building a high performance slant (or any engine) and think you might be trying different cam installed center lines,
Check for proper valve to piston clearance 5 or 6 degrees either side of your starting point. Easier to make corrections during the initial build, then later, or running out of clearance and bending valves, because you didn't check. If I remember correctly, advancing the cam gives less clearance on the intake, and retarding the cam gives less clearance on the exhaust.
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