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In addition to above query - another tech question:
I bought the TRW forged aluminum pistons, whereas OEM are cast. We’re still talking cast iron cylinders and aluminum pistons, so I assume that I’ll want the machinist who will bore my cylinders out to .030” to match the pistons should set the fit up at least in the middle of the spec—i.e., not too tight, to avoid seizing. In other words, is there any difference between cast and forged pistons in terms of allowance for expansion?
Forged pistons expand more than cast pistons. Generally, cast pistons were usually set up at .001-.0015. Forged at .0035-.004. In hotter areas, like Texas and New Mexico I'd go with the loser fit and let the engine warm up to the touch enough that you don't want to leave your hand there long before accelerating very hard. You may have a little piston noise on cold days with forged pistons.
If there are numbers on the underside of the piston, see if you can find the specs form TRW
TRW & Wiseco forged pistons in the old bikes tend to gall the skirts if they are set up on the tight side with the old cast iron cylinders, do a little google foo on this topic. Unless it's a dedicated hotrod I'll use cast in the pans & shovels. These new manufacture or new old stock from somewhere? the newer low expansion TRW, I've heard .0035-.0045 in hot area's, the old stock forged, I'd start at .0045 min.
Thanks for the responses. I had imagined that forged pistons were a better choice (more durable) but now am wondering whether I should send them back to BMI, as perhaps I’m needlessly complicating my task…?
I will be running mostly (if not exclusively) in New Mexico and surrounding states—with maybe a jaunt out to the Pacific if I get ambitious. I tend to run my ’84 Beemer into the city for groceries when it’s too cold for an unfaired bike, Nov-Feb. So, hotter weather is what I want to anticipate in setting the cylinders up.
As for these pistons: BMI doesn’t have much info on their website—but when they get here, I’ll see if there are markings on them or literature in the box that clears up whether they’re NOS or the newer, low-expansion versions.
As to the specs & setup: if I were to keep them, sounds like .0045” might be a good figure (thank you).
If I do keep them, I will relate your advice to the longtime owner and shop foreman of Motor Machine here in ABQ, who would be boring the old cylinders. He does mainly auto hot rod and racing engines, but said if I made him a fixture he’d be happy to help me out. I’m hoping this fixture I made to hold them for cross-hatching last time around might be sufficient to let him hold them firmly for boring…? I could easily add pieces of angle extending outward from the base on both ends if easier for him to clamp down…?
Thanks for the responses. I had imagined that forged pistons were a better choice (more durable) but now am wondering whether I should send them back to BMI, as perhaps Im needlessly complicating my task ?
I will be running mostly (if not exclusively) in New Mexico and surrounding stateswith maybe a jaunt out to the Pacific if I get ambitious. I tend to run my 84 Beemer into the city for groceries when its too cold for an unfaired bike, Nov-Feb. So, hotter weather is what I want to anticipate in setting the cylinders up.
As for these pistons: BMI doesnt have much info on their websitebut when they get here, Ill see if there are markings on them or literature in the box that clears up whether theyre NOS or the newer, low-expansion versions.
As to the specs & setup: if I were to keep them, sounds like .0045 might be a good figure (thank you).
If I do keep them, I will relate your advice to the longtime owner and shop foreman of Motor Machine here in ABQ, who would be boring the old cylinders. He does mainly auto hot rod and racing engines, but said if I made him a fixture hed be happy to help me out. Im hoping this fixture I made to hold them for cross-hatching last time around might be sufficient to let him hold them firmly for boring ? I could easily add pieces of angle extending outward from the base on both ends if easier for him to clamp down ?
In addition to the above, re: using forged vs cast:
This video shows an issue I didnt know aboutmaking sure with a different style piston that the skirt isnt coming down over the oiling hole in the cylinder. The piston skirts look similar to the cast versions I've got now, but I guess Id better measure them to be sure Im good to go.
I will be using regular James paper base gaskets with a thin coat of Ultra Gray, as my cases are a little chewed up @ the rear by that L-shaped oil drainage trough, and they leaked with Cometic metal gaskets but sealed up nicely last time with James + Ultra Gray. I will be using .040 Cometic MLS head gaskets, as they worked great last time around.
Also: not only are the Kibblewhite valves and the seats I cut looking fine, but my S&S roller rockers are intact. I random-checked a rocker, and the .008 end-play that I established in the original build by machining the rocker shafts is still spot-on with the nuts torqued to 15 ft-lbs, moving freely and smoothly. So, dont have to mess with the rockers, thank goodness
What I’m seeing confirms what TwiZted said (no surprise there, obviously): a modern forged (low expansion) Harley piston will run quietly, with the rings lasting a long time, if set up with .0045” clearance.
Update: now I’m seeing .006” and even more recommended for TRW forged pistons. Hmmm… this may not be worth the headaches.
TwiZted: one poster who has used them said that a.016" gap for the top two rings is about right. Is that what you’d recommend? For some reason, I had .012” in my head.
Paul: I don’t mind letting her warm up a little while before heading out. (The Beemer airhead is so cold-blooded, I have to go inside for coffee before it’s ready to ride.)
Last edited by NM Pan-shovel; Apr 12, 2025 at 11:03 AM.
Took me a minute to figure out a method of removing the ‘key ring’ style wrist pin retainers. There’s no notch in aftermarket pistons, so you can’t use the special tool I stupidly bought a couple years ago to R&R them for removal; just for installation I guess…
Anyway, I was able to lift the tapered end of the clips with a teensy screwdriver, then lever & bend them out enough to grab with needle nose pliers, and voila.
The pins were badly galled from oil starvation. Had to really muscle the puller but got them out.
Looks like the front rod bush wasn’t installed properly; the oil slot isn’t lined up well at all. However, they’re both toast. Luckily, I had the foresight to add new ones to the gaskets order—which doesn’t get here until Tuesday.
I am hoping that I can machine a stepped disc to fit my wrist pin puller, in order to use it to R&R the rod bushes. We’ll see…
obstructed passage...
roasted bush...
open passage...
a matched set...
Last edited by NM Pan-shovel; Apr 12, 2025 at 08:18 PM.
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