21 replies to this topic

[skoughi]

One more question chaps, before committing to buying my pistons I'm wanting to know that my bores are within tolerance.The bores have been measured by a trusted engineer and he says they're fine, but I'd like to know what they are before I buy the pistons. The bore have been given a light hone but after cleaning I feel that a little more honing wouldn't go amiss as there are visible vertical scratches on the bores, which I can't feel with me fingernail! I've searched on here and found that everyone advises to go for a .003" gap between piston and bore and that the piston is measured dowm at the pin, but I don't have the pistons yet to measure. So calling on everyones experience what would be the maximum bore size I can go to for these 21253-60 pistons? I've tried calling Minispares and asking them what the technical info on these pistons and rings are but they couldn't help. You guys are awesome bytheway!

[ACDodd]

What is the actual bore size? The temperature it was measured at is also useful? Why can you not get it accurately measured?
You need top middle and bottom, also inline with the crank and 90degrees?

Ac

Edited by ACDodd, 07 September 2015 - 05:32 PM.

[skoughi]

I've obtained a bore gauge for a few evenings so will give it a go and measure up. I have only got this now as I've been getting a few freebie jobs done by a very helpful engineer, I didn't want to puish my luck with him! Also I like to try things myself and hopefully learn something along the way. The temp in my garage tonight is probably a warm 6-9 deg, how much of a difference will this make to any measurements I make compared to say 16-19 deg?

[Spider]

Given the choice, I wouldn't fit new pistons to an 'old' bore, in fact, I'd have to be very hard pressed to do so. The Piston sizing (on the box) is nominal, not exact. While just about every piston you'll pick up these days is pretty good as far as sizing goes, there are small variations between them.

 

Or to look at it another way, any reputable machine shop will not bore your block until they have the actual pistons to be fitted in their hands to measure and bore from, the better shops will even bore and hone each bore for a particular piston, thus matching them.

 

I did a write up here on this a little while back, I'll see if I can find a link to it.

[skoughi]

I take your point Moke, I'm simply hoping that the bores will be suitable for direct replacements due to my inexperience. I guess going by this my only option now is to buy the next size up pistons and get the block bored out for 1360. Just to add to my now rather depressing evening the bore gauge I've borrowed seems to have a dial gauge that's jammed and won't read any difference. I'm unsure now as to what to do, I guess I could go to the machine shop and pay them to thoroughly measure the bores to see where I'm actually at with this. A rebore to the next size isn't the end of the world as I have to buy new pistons anyway, thanks for all your advice and help guys.

[Spider]

Here's that promised Link;-

 

http://www.theminifo...gine-machining/

 

OK, maybe I'm a bit (pardon the expression) anal as to how I go about these things, but maybe looking at the cash and effort involved in the totality of what you are doing, which will be at a rough guess in the region of 400 quid plus, then the added cost of a rebore & hone will only be a few tens on top. Then it's all new and you'll get the best possible life from the rebuild. In the early days, I did a few 'budget' rebuilds (hone & re-ring) and they were OK but I never got anything like the life from them compared to a full rebuild.

 

Please, no disrespect what so ever here, but a dial bore gauge I recon is not an instrument you can just pick up and use out of the box without experience in using them. You also need a Micrometer to set it to. I use the said Micrometer to measure the Piston, then add to that the piston to bore clearance that I want, and lock it off. Then zero the Dial Bore gauge to that. There's probably a zillion ways you could do this, but that's just what I do. I do that for each piston separately and number the pistons.

 

Chin up mate, you are asking all the right questions.

[skoughi]

Cheers, I have no problems asking questions as long as there's someone willing to give an answer! I fully understand what your saying about the bore gauge, I watched quite a few online videos on how to set it up which was straightforward enough but as you rightly pointed out even knowing how much to tighten the micrometer part of it to give a true reading can only come with years of experience and guidance. Sometimes I can take on too much! I want this build to be as good as possible with the experience and ability I have, if it can run and will burn less oil than petrol then that will be a miracle! However I have to except that there are things I can't do and properly measuring the bores is one of them. I've decided to go and actually pay the local machine shop to do a proper measure of these bores, only then will I know if I can chance getting away with the bores as they are. I had time to check down two of the bores with and old piston ring and feelers, I measured slightly down from the top then at 1" below this then 2" then 3" and was getting a difference of .001, but I obviously don't still know the actual bore size. I'll go discuss and see what results I get back then I'll let you know what we came up with. Thanks again for the help.