cylinder bore lapping

[william cubbonParticipant @williamcubbon74452]

I have a Milford Mite 1.4cc cylinder now built into a petrol 2 stroke. Poor compression requires the bore surface to be lapped and a new piston to match.

What lapping compound would the forum recomend, ie grit size. The steel cylinder is hardened.

Thanks for any response.

Patrick Cubbon

[william cubbonParticipant @williamcubbon74452]

lapping compound

[JasonBModerator @jasonb]

I use silicon carbide powder mixed with a light oil, 1000g is the finest I use to lap the piston into it's bore.

Ramon's long thread here on MEM is worth a look

[william cubbonParticipant @williamcubbon74452]

Thanks Jason for promptness. Long thread is really good. Am now reminded of Ramon's ME articles in 2014 in particular ME No 4484. 1000g SC, you recomend, will now enable me to purchse on line – thanks. Patrick Cubbon.

[JasonBModerator @jasonb]

You may want something coarser than that to start with when lapping the cylinder say 320 then 600

[Oily RagParticipant @oilyrag]

Just a question of prudence here – would it not be best to lap the bore using a dummy piston (I'm thinking here of, say, a brass bar turned to a suitable oversize ) and then running the piston in to the lapped bore? After all the bearing points for the piston will be the ring(s) and the lower piston skirt. Is the piston aluminium, CI or steel?

I would point out here that I have no knowledge of the 'art of model engines' so am willing to learn something new here; so if my point is stupid – don't be afraid to say!

Martin

[JasonBModerator @jasonb]

Usual practice is the cylinder is lapped first, the piston lapped and then finally the piston lapped into the cylinder.

No rings, CI piston runs direct in the cylinder.

The first 5 pages of the link I posted cover lapping the cylinder before moving on to the piston. William also said lapping the bore and I said lapping the cylinder.

Last post on this page and the next couple on page 2 show the process on my Firefly engine.

 

Edited By JasonB on 28/07/2021 19:55:34

[Oily RagParticipant @oilyrag]

Thank you Jason – one is never too old to learn something new every day!

The only experience I have of model engines is as a schoolboy with a 1.5cc ED diesel – still got the three scars on the back of my hand where the prop caught me! Other than that we had a 2.5cc piston /rod and barrel assembly modified with a reed valve head as an air compressor on an F1 engine in the early '90's (the air was used to supply the air springs for the valve mechanism – excess air was bled off to pressurise the coolant system to 6bar.

Martin

[ZanParticipant @zan]

Try some “Time saver lapping compound “ expensive, but outstanding performance and it’s non embedding so will not cause any long term problems. Google it

Edited By Zan on 29/07/2021 18:09:14

[william cubbonParticipant @williamcubbon74452]

(I used Message member, but notice I can also post a reply which will now do.!)

Thanks for your lapping suggestion, Google price is ok for workshop budget. Silicon carbide should correct 15/32ins diam cylinder bore surface condition suffering from uneven wear and possible distortion, the cylinder material is hardened steel. A new piston will of couse also be required.

[Ramon WilsonParticipant @ramonwilson3]

William,

As the liner is hardened you may find diamond paste more suitable and efficient though I have used silicon carbide to reclaim hardened liners

It's not a good idea to use a diamond product on unhardened parts without an ultra sonic cleaner as the diamond embeds itself which normal solvent based cleaning won't remove entirely so wouldn't use it to lap the piston

I'm intrigued – a Milford Mite converted to 'petrol' – spark ignition?

Regards – Ramon (Tug)

[bernard towersParticipant @bernardtowers37738]

Tug, Didn’t think it would be long before you spotted this one!!!

[Ramon WilsonParticipant @ramonwilson3]

Hello Bernie, long time no see. Hope you are well in all of this.

Yes always happy to help if I can on lapping though I don't think I'll be doing much more – beginning to think about down sizing in the next year or two.

Another year without Forncett I see – perhaps next year eh?

Best – Tug

PS Apologies for encroaching your thread William

Edited By Ramon Wilson on 30/07/2021 18:51:34

[Ramon WilsonParticipant @ramonwilson3]

Patrick (William) I have replied to your PM.

Tug

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