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BRM's Flexible Honing, Surface Finishing, and Deburring Blog Blog

BRM's Flexible Honing, Surface Finishing, and Deburring Blog

BRM's Flexible Honing, Surface Finishing, and Deburring Blog is the place for conversation and discussion about how to solve difficult finishing problems. For over 50 years, Brush Research Manufacturing (BRM) has helped customers use brushing technology to clean, rebuild, and resurface components ranging from engine cylinders to brake rotors to flywheels to firearms. BRM's Blog on CR4 provides real-world examples of how flex hones and wire brushes work. It also evaluates related technologies and invites questions from the community.

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Flex-Hones, Valve Guides, and Engine Compression Ratio

Posted August 16, 2011 4:55 PM by BrushResearch

Cool cars include "hot hatches", high-performance versions of hatchback automobiles. During the 1980s, the French carmaker Peugeot built what some automotive observers regard as the greatest "hot hatch" of all time: the 205 GTi 1.9L. Early versions of this supermini needed tuning, however. The in-line four-cylinder XU9J1/Z DFZ engine offered excellent throttle response, but the compression ratio was a mere 8.4:1. As Adelaide machinist chez00 posted in Aussie Frogs: The Australian French Car forum, a Series 1 DFZ that he purchased left him "wanting in power".

Naturally Aspirated vs. Forced Induction

Increasing the compression ratio with forced induction wasn't an option, however. "I love the sound and throttle response of a nicely-tuned aspirated engine," chez00 explained. With some help from his Aussie Frogger friends, chez00 learned that the Series 3 Peugeot 205 GTi 1.9L featured a higher-compression engine. With its smaller chambers, the cylinder head raised the compression ratio to 9.2:1. Even better, by bolting this cylinder head to the bottom-end of his DFZ, chez00 could boost the engine compression ratio to 10.5.1 because of the smaller dishes in the pistons of the DFZ.

Valve Guide Replacement

In an excellent series of posts on Aussie Frogs, chez00 describes this high-performance cylinder head build. First, the automotive machinist disassembled the Series 3 cylinder head that he purchased from a fellow Frogger "to get a closer look at the condition". After pressing out the valve guides, he matched up the inlet and exhaust gaskets with the ports. Although some of the inlet valve guides were "passable," chez00 opted to fit all new ones because there was "a little too much play" in the exhaust valve guides.

Valve Guide Reamer or Flex-Hone?

In high-performance automotive applications, guide clearance is crucial. Too much clearance between valve stem and valve guide can cause oil to slip into the combustion chamber, resulting in clogged spark plugs and loss of power. Too little clearance prevents the passage of oil between guide and stem, with friction-induced failure of piston, valve, and valve guide all possible. Selecting the right tool for the job is critical then, especially because a ridge reamer "can leave a rough finish inside the guide," as chez00 explains.

As part of his automotive restoration project then, chez00 used a diamond flex hone for surface finishing the inside diameter (ID) with "a beautiful 'crosshatch' much like that in a newly honed piston bore." This cylinder hone enabled the machinist to finish the exhaust guides to .0022-in. clearance and the inlets to .002-in. "This allows the guide to hold oil while still having a relatively tight clearance," he explains, adding that "it is small things like this that makes the difference between an average engine and a performance one."


Aussie Frogs: The Australian French Car Forum

Peugeot 205

Increasing Compression Ratio

Editor's Note: CR4 would like to thank Brush Research for contributing this blog entry, which originally appeared here.


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