Piston and ring size markings

[Laverda Dave]

Hi Everyone,
The rebuild of the 400/4 I bought in boxes is progressing well with the engine cases now back from aqua blasting and various essential parts purchased from Mr Silver's parts emporium.  I have also collected the head from J D Autoworx following a 7 thou skim, valve seat cuts for the four new inlet valves and two brass inserts being installed following the removal of the broken exhaust studs on No.3 cylinder.
I dropped the frame off today to have it checked, its just a shame the previous owner had it blasted and powder coated without checking the alignment first.  I am hoping a hot torch isn't going to be required to straighten it if it is bent otherwise it will get a bit messy! A lesson here is that any frame should be checked before stripping and painting especially considering the age of bikes we ride and the probability of the bike having at least one excursion down the road! A Morini I restored a couple of years ago had a perfect looking frame and I was going ahead to paint it but though a check would be a good idea. I'm glad I did, it was 2 degrees out at the steering head!
Anyway, the reason I'm here.........I have been checking the pistons and rings today, plan is to fit a new set of rings only as the previous owner measured the bores and found very little wear and virtually no oval wear at all.  My question is, how do I know what size pistons and rings I have without having access to specialist measuring equipment? I thought the pistons or the rings would be stamped with either 'STD' markings or 0.25 etc for any oversize.  I cannot find either on the pistons or the rings?
Any advice would be appreciated.
I will post a few pics on the site of the rebuild as it progresses when I a chance and have clean hands!
Dave

[Bryanj]

Unmarked pistons are normally, but not always, standard. The rings are marked on the top face by the join again no markings are standard BUT I have seen rings worn so much that the markings have disapeared

[paulbaker1954]

Have you measured the bores? This is best way to check but FYI I just rebuilt my 4 engine and the Pistons had no markings except the forward arrow and these were standard size

By the way I have a set of std rings if you need and want to make me a sensible offer

They are NOS rings by the way

[Laverda Dave]

Thank you both for the advise.

Very helpful, I thought they might be standard as my old 400/4 had 0.25 stamped on the rings when I had that re-bored.

The guy who began the project measured the bores (he's an engineer and has top spec measuring equipment).  He told me the bores and pistons are well within tolerance and sent me a copy of the measurements (attached). I am not an engineer on that level and to be honest I had some difficulty reading the attachment. From what I can see all appears well, the pistons show no sign of blow by although they do have evidence of some scuffing but nothing to be worried about (maybe a photo would help?).
I was just going to put a new set of rings in but............ If anyone reading this thinks the measurements given indicate a re-bore is needed I will take this route. I would like to say the bike has done 47000 miles but......it had a CB250 speedo fitted so any true mileage is guesswork and the engine was in bits and stored in several carboard boxes (I love a challenge........!)
Paul, If all is well and the general opinion is to stay with what I have I will buy those rings from you.  Let me know how much and your details, mail me at davejotaellis@talktalk.net with your price and we can go from there.
Regards
Dave

[Bryanj]

If standard bore is 51mm (sorry havn't got that book to hand) you are good to go with new rings and a glaze bust on the bores

[paulbaker1954]

Something very odd here, the standard spec for bore size for the 500 four is 56-56.01mm with a servicable limit of 56.1mm

Youer measurements indicate 51mm which makes no sense (assuming it is a CB500 four.

[Chris400F]

First line of the first post says it is a 400/4, not sure where the 500 four came from ....
51mm would mean the bore size is standard.

[paulbaker1954]

Ahh silly me I didn't spot that 

[Laverda Dave]

Well, the rebuild still gathers momentum although another evening spent in the garage following the glaze bust has revealed a nasty surprise on the piston and bore front.
I thought I'd measure the gaps on the old standard rings after inserting them in the bore about 25mm from the top of the individual bores and before ordering some new ones from DS.  Shock and horror  , I appear to have some heavy wear on the bores. 
The measurements I took going from number 1 to number 4 are: 0.66, 0.90, 0.96, 0.92.  These gaps appear big enough to drive a bus through  !

I am further confused by the wear limit measurements given in the Haynes manual as being between 0.15-0.35(mm) whilst the Clymer manual gives a wear limit of 0.15(mm) with no upper or lower limit? I'm not sure what measurements I should be taking notice of, fair to say though it would appear I need a re-bore. 

When I collected the barrels from the engineer who did the glaze bust he advised the piston to bore clearance when measured was 25 thou, he thought that was a bit slack but OK (old Triumph's apparently run 40 thou clearance as standard).
In view of the gaps I measured it would appear as though I need 0.75 oversize pistons, nobody appears to make this size, DS, Yamiya or Cruzinimage, I might have to go to 1mm oversize.  The Yamiya parts appear to be cheaper than DS although I will likely be stung for import tax making them more expensive.  Has anyone any recommendations?

Dave

[Bryanj]

You have just used an old worn ring to measure a bore THAT DON'T WORK, you need to use an internal micrometer or a cylinder bore gauge

[Laverda Dave]

Thank you Bryan. Yep you are right, that thought entered my head at about midnight last night when I couldn't sleep! Of course the rings are designed to wear and I measured worn rings in a glaze busted bore...oops .
Anyway, the good news is I finally had the opportunity to use a brand new Helios bore gauge I bought about 20 years ago at an engineering tool sale to measure the bores this evening.
I took six measurements of each bore front to back, side to side, top, middle and bottom of each cylinder.  The maximum bore measurements from 1- 4 were:
1 = 51.63
2 = 51.64
3 = 51.64
4 = 51.64
The measurements were the same throughout the bore but I would expect that as they were all glaze busted only last Friday so should be the same!  The engineer didn't have the pistons when he used the glaze buster but did advise me the piston to bore clearance would be about 2.5 thousand if it was a standard piston (it is) following the glaze bust.  The Haynes manual states a maximum piston to bore clearance of 0.050mm. He was also unable to remove the wear marks at the top of each bore (about 7mm of the bore is untouched by the glaze bust at the tops).  This is because the engineer didn't want to remove too much material.

I compared the measurements with the Haynes manual specs that states ' The difference between the lower bore measurement and the top bore measurement should not be greater than 0.12mm, if it is the cylinder requires a rebore'.  Not much use to me as the measurements are the same at the top and bottom due to the glaze bust!
The Clymer manual advises the cylinder wear limit is 51.1mm.
Going on the Clymer wear limit I'm not sure if I need a rebore or not, by my calculation I am up to 0.36mm below the maximum wear limit or I could be wrong and actually be 0.54mm above the wear limit and require a 0.75mm rebore although I will have to go 1mm oversize as 0.75 pistons are unavailable 
My head hurts after all this 
Dave

[Bryanj]

If standard bore is exactly 51mm and your measurements are accurate you are 0.14mm(5.6 Thou) worn on a 0.5mm oversize bore

[Laverda Dave]

Thanks Bryan.
So, if I'm already worn 5.6 thou on a 0.50 oversize bore I'm going to need a rebore of 0.75 thou oversize then. The problem here is I am unable to find anyone WHO has any 0.75 oversize pistons despite going through the usual suspects including DS and Cruzinimage. This means going the full 1mm oversize then. There is only one set of pistons I can find as 1mm oversize made by IMD on e-bay. Has anyone had an experience with these pistons or better still, does anyone have a spare set of 0.75 pistons they want to sell?
I did think about going the 460 big bore route but that would entail a whole host of other work so that's a no go.