Hello!

I want a new engine pistons. The engine is very worn. Service manual, the maximum 0.25 may be oversize. Tuner larger pistons from (0.50). They should put in the pistons? What could be a problem due to the large oversize?

Thx.

 

Yes 0.25 is the most you can bore out our stock sleeves, if you want to go farther than that you will need to get the block sleeved. Do a search and you will be able to find the different sleeves, and places that can do it for you.

 

+0.25mm is the maximum recommended workshop manual oversize specification.

Stock blocks have been successfully bore/honed out as far as 88mm (+1mm) without any requirement to sleeve the block. Mahle is the current best option for FRM compatible pistons.

 

Could you elaborate on the honda honing tool that can be used to hone a cylinder from 87.00mm to 87.25mm.

I've only even seen people using specialised honing machines to achieve a +0.25mm increase in bore diameter accurately.

 

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I don't feel comfortable when cylinder walls are polished. Honing is a process where a rough surface pattern provides oil film for the piston rings. The rings are "surfing" on the oil film and for adhesive reasons there must be some small grooves on the surface.
Honda describes the Honing procedure clearly inside the manual. And there is a crosshatch pattern recommended. BTW, the procedure is nearly the same as for ALUSIL or NIKASIL cylinders. The Tools and steps to do the Honing for the FRM cylinders are no miracle or specialized for our engine.

Also, you cannot wear more than 0.02mm with honing. It is therefore impossible to reach 0.25 more bore just by honing. The cylinders have to be bored by machining. After that process, honing will be done for providing the right roughness for the surface.

The FRM layers inside the cylinder walls are min. 2mm thick. Boring up to 88mm is no problem. You just have to look for enough remaining sealing surface at the head gasket.

 

Yes understood, that's why I stay with the OEM recommendation.

.25mm is ten times more than .025mm.

 

The main requirements when honing FRM bores is to use appropriate stones in a hard hone that will not load up with material. If material sticks to the stones it can drag on the bore leaving deep scratches. Typically friable stones are used so the surface of the stone is shedding material and continually exposing new material.

The final surface finish ( grit spec ) is also much finer than that typically used for conventional bores. There is still some cross hatching to hold oil, only much finer than normal engines. The workshop I use finishes with a planishing/polishing stone that skims and high spots off (at a microscopic level).

Getting back to the original posters question.

If your block is currently 87.00mm and there are no deep scratches in the bores it will be able to be bore/honed to the factory oversize of 87.25mm. Just make sure to use a workshop that can work with FRM bores. Also pay close attention to the bore clearance as they are very tight in these motors. Factory +0.25 pistons will fit your rods.

Alternatively, look at the new Mahle +0.25 oversized pistons. These have the advantage that you can select compression ratios. There is one difference that the pin size is exactly 23mm they wont fit standard rods. You can have the pin ends of the rods resized, but the cheapest option is to fit a set of Brian Crower rods with the correct pin size and bush in the pin end.

Still on FRM pistons. 88mm pistons are available from Woessner, and hopefully soon off the shelf from Mahle.

Good luck with the build.