Groves

Hey guys, im currently building up my h22a. ive
come to the conclusion that i wont be boring/honing my motor since the general consensus is that its not necessary if your cylinder bores are smooth and have no scratches up n down the cylinder. mine are in very good condition and im having a hard time finding a shop within 50 miles of me familiar with FRM anyways. but anyways, i have BBBB sized pistons which are 86.890-86.993mm in size and am curious to know if i just go to 87mm piston (specifically nippon racing type s pistons 87mm) and gap the rings would i be good to go? wouldnt the piston to wall clearance be way to tight? this is my first motor build and its quite the learning curve. especially with FRM.

Aradin

You should be fine with the Nippon pistons. The aftermarket in cast pistons usually only offers a one size fits all sort of deal. They will be a touch looser on some engines but usually not enough to make a noticeable difference. Just be sure to check the piston to wall clearance during assembly.

Groves

well they offer 87, 87.25,87.5,87.75,88 i was originally goin 87.25 but i explained why im not above. are you saying the 87mm size should be good but just double check the piston to wall clearance?

Aradin

If your block is still 87, aka has never been bored/honed, then yes get the 87mm and it will be fine.

Groves

bump if anyone else could chime in and give me some knowledge lol.

Aradin

If you really want to try to figure out your exact piston to wall then you could always contact Nippon and find out the exact size of their 87mm pistons. To elaborate and clarify on what I said above, standard bore stuff has the piston to wall clearance built into the piston and not the bore. Whereas if you go oversize, it's up to the machine shop to set the piston to wall. Obviously this creates a scenario when using stock bore stuff that the piston to wall is not 100% correct. Most aftermarket manufacturers don't want to make 2 separate stock bore pistons and account for this by making the one piston set that will fit engines that required either A or B pistons. If you had an AAAA engine, then the piston to wall would be slightly loose. Since you have a BBBB engine, the stock bore Nippon pistons should fit well. They are cast pistons anyway which do not expand very much and thus don't require a large piston to wall gap like a typical 2618 or 4032 alloy forged slug.

FRM sleeves are a very hard material that generally do not wear away or get looser over time under normal circumstances. If you want an exact fit then you would have to source another set of OEM BBBB pistons to match your engine.

You can roughly measure the piston to wall with just a long set of feeler gauges if you don't have a dial bore gauge or other preferred measuring tools. You could purchase the Nippon pistons and measure your clearances. If you don't like what you find when you measure then you can always return them. I hope this better answers your question.

Source: I've been building engines for a decade and worked in a machine shop for a long time.

Groves

thank you for all the good info. if you have been building engines for a long time i assume youve built some h series motors a few times. say all the measurements are good on the standard size 87mm nippons do you recommend just popping in the pistons, gap the rings, and go. or do you recommend i have it bored/honed to 87.25mm and just get the 87.25 nippons.

Aradin

Given the fickle nature of the FRM material I would just stay 87. Check clearances, gap rings and go. There's a very good chance an average machine shop will damage the sleeves or incorrectly hone them when trying to machine it if you were to overbore.